fundamentals

Question 1

((false && undefined) || (false || undefined));

Answer 1

undefined

Question 2

What does the code return?
[…Array(3)]

Answer 2

[undefined, undefined, undefined]

Question 3

Insert an array into another array with spread syntax

let foo = [1, 2, 3]
// code here
bar; // => [1, 2, 3, 4, 5, 6, 1, 2, 3]

Answer 3

let bar = […foo, 4, 5, 6, …foo];

Question 4

Which operator converts the non string to a string if the other operand is a string?

Answer 4

+

Question 5

What does the code do?

Array.prototype.splice(0, 1) ;

Answer 5

It removes 1 element from energy, starting at index 0

Question 6

What does this return?
‘4’ + 3

Answer 6

3 is coerced to the string ‘3’, so the result is ‘43’

Question 7

What does this return?
‘fava’.charCodeAt(2)

Answer 7

The number of ‘v’ in the unicode: 118

Question 8

undefined >= 1

Answer 8

// false – becomes NaN >= 1

Question 9

What does this return?
1 + true

Answer 9

true is coerced to the number 1, so the result is 2

Question 10

What happens here?
[1, 2] * 2;

Answer 10

[1, 2] becomes ‘1,2’, * 2 then NaN * 2 => NaN

Question 11

NaN != NaN

Answer 11

true

Question 12

When should you use try/catch/finally ?

Answer 12

A built in function or method can throw an error and you need to catch it

A simple guard clause is impractical

Question 13

Operator after operand: (a++)
a = 1;
What is:
b = a++;

Answer 13

equivalent to “b = a; a++;”. so now b is 1 and a is 2

Question 14

null == undefined

Answer 14

true

Question 15

Braket notation is?

Answer 15

An operator. Not a method

Question 16

‘11’ > 9

Answer 16

// true – ‘11’ is coerced to 11

Question 17

Does javascript interpret a statement starting with a curly brace as an object literal?

Answer 17

No. it thinks it’s a block. Use parenthesis to force it to take it as an object literal

Question 18

What does this return?
const str1 = ‘strunz’;
str1.endsWith(‘u’, 5);

Answer 18

false

Question 19

How do you count all of the properties in an array, as opposed to only the indexed values?

Answer 19

Object.keys(array).length

Question 20

Describe a mental model for how closures work

Answer 20

At the time of the function definition, javascript collects all the variable names that are in the function scope and places them in an ‘envelope’. This is then assigned to the function object. These variable names are pointers to the variable, as opposed to the value the variables point to. This is so that javascript can be aware of any reassignments the variables may have

Question 21

What does this return? Why?
‘50’ < ‘6’

Answer 21

true. ‘5’ < ‘6’
There is no coercion if same type

Question 22

Merge objects with spread syntax

let foo = { qux: 1, baz: 2 };
let xyz = { baz: 3, sup: 4 };
// code here
obj; // => { qux: 1, baz: 3, sup: 4 }

Answer 22

let obj = { …foo, …xyz };

Question 23

Create a shallow clone of an array with spread syntax

let foo = [1, 2, 3];
//…. code here
foo.pop();

console.log(foo); // [1, 2]
console.log(bar); // [1, 2, 3]

Answer 23

let bar = […foo];

Question 24

How to you return 2 to the power of 3?

Answer 24

Math.pow(2, 3)

Question 25

(‘a’ && (false || undefined) && ‘’);

Answer 25

undefined

Question 26

How to check if a value is an integer?

Answer 26

Number.isInteger(value) => true or false

Question 27

Name four side effects?

Answer 27

reading from the keyboard (prompt()

generating random numbers

accessing date and time

If a function can raise an exception and doesn’t catch and handle it, it has a side effect:

Question 28

undefined == null

Answer 28

true

Question 29

How do you make a quantifier lazy?

Answer 29

Add a ‘?’ to the quantifier. Ex. *?

Question 30

Operator before operand: (++a)
a = 1
c = ++a;

Answer 30

equivalent to “++a; c = a;”. so now c is 2 and a is 2

Question 31

What does the code log and why?

console.log(greeting);
let greeting = ‘Hello world!’;

Answer 31

ReferenceError: Cannot access ‘greeting’ before initialization;

Variables declared with let and const are hoisted but not initialised. They are said to live in the TEMPORAL DEAD ZONE until they are initialised to a value

Question 32

What is a type error?

Answer 32

Trying to access a value on a property that doesn’t have any such as null.
When you call an method on an object you are accessing a value (the method) on that property

Question 33

What does the regex specify?
p{3}

Answer 33

Exactly 3 occurrences of p

Question 34

Perform Object Destructuring on the following code:

let obj = {
foo: “foo”,
bar: “bar”,
qux: 42,
};

that does the same as:

let foo = obj.foo;
let bar = obj.bar;
let qux = obj.qux;

Answer 34

let { foo, bar, qux } = obj;

Question 35

What does the code log and why?

console.log(greeting);
var greeting = ‘Hello world!’;

Answer 35

undefined. var is hoisted but nothing is assigned to it. It’s like it was written like this:

var greeting;
console.log(greeting);
greeting = ‘Hello world!’;

Question 36

Statements vs expressions:

Answer 36

Unlike expressions, statements don’t resolve to a value:
let yay;
they control the execution of a program: if else statements, for loops etc.
Statements help you do something as opposed to giving a value

Question 37

What does the code log?

let ocean = {};
let prefix = ‘Indian’;

ocean.prefix = ‘Pacific’;

console.log(ocean);

Answer 37

{ prefix: ‘Pacific’ }

Question 38

What are the associations between a name (or key) and a value in an object?

Answer 38

properties

Question 39

Can Array.prototype.slice be passed a negative index as an argument to extract elements from the end of an array?

Answer 39

Yes:

[‘f’, ‘a’, ‘v’, ‘a’].slice(-2, -1); => [“v”]

[‘f’, ‘a’, ‘v’, ‘a’].slice(-2); => [“v”, “a”]

[‘f’, ‘a’, ‘v’, ‘a’].slice(-2)[0]; => ‘v’

Question 40

What does this log?

let person = {
title: ‘Duke’,
name: ‘Nukem’,
age: 33
};

console.log(Object.entries(person));

Answer 40

[[‘title’, ‘Duke’], [‘name’, ‘Nukem’], [‘age’, 33]]

An array of sub arrays

Question 41

What does this return?
0 == false //

Answer 41

true – becomes 0 == 0

Question 42

What is an assignment?

Answer 42

a stand alone expression that assigns a new value to a variable

Question 43

What does this return?
const str1 = ‘strunz’;
str1.startsWith(‘u’, 4);

Answer 43

false

Question 44

11 > ‘9’

Answer 44

// true – ‘9’ is coerced to 9

Question 45

slice() vs splice()

Answer 45

slice() return a portion of an array. splice() can modify an array; delete items, insert them or replace them
splice(start, deleteCount, item1, item2, itemN)

Question 46

Atom shortcuts: Highlights all instances of the same word

Answer 46

command control g

Question 47

What does the regex specify?
p{5,}

Answer 47

5 or more occurrences of p

Question 48

how do you get the unique values from an array?
let array = [1, 1, 2, 3, 3]

Answer 48

[…new Set(array)]
=> [1, 2, 3]

Question 49

What does this return?
1 + undefined

Answer 49

NaN

Question 50

const animals = [‘ant’, ‘bison’, ‘camel’, ‘duck’, ‘elephant’];

console.log(animals.splice(0, 2));
console.log(animals);

Answer 50

[“ant”, “bison”] // the deleted items
[“camel”, “duck”, “elephant”] // the mutated array

Question 51

Which data type does Number.prototype.toFixed() return?

Answer 51

A string

Question 52

Return a random integer between 0 and 3

Answer 52

Math.floor(Math.random() * 4);

Question 53

What does the code return?
[…Array(3)].fill(2)

Answer 53

[2, 2, 2]

Question 54

JavaScript style guides usually recommend using which notation when possible.

Answer 54

dot notation. as opposed to bracket

Question 55

What is an initialiser?

Answer 55

Looks like assignment but it’s different in terminology; it’s the expression to the right of ‘=’ in a variable declaration. ex:
let number = 35;

Question 56

What is this equal to?
[ …unused, …washed ]

Answer 56

unused.concat(washed);

Question 57

((false || undefined) || (false && undefined));

Answer 57

false

Question 58

Why is it a bad idea to run arithmetic operators on arrays?

Answer 58

Besides producing useless results they run without producing a warning!

Question 59

What specific examples of code can you think of that strict mode doesn’t allow?

Answer 59

Cannot create undeclared variables in functions so that they become global variables

Cannot start an integer with 0. Apart from 0 itself

Question 60

Objects are containers for two things:

Answer 60

data (properties) and behaviour (methods)

Question 61

undefined == ‘’

Answer 61

false

Question 62

What does typeof null return and why?

Answer 62

Object. This is a historic bug. It’s too late to change it

Question 63

How to get a value from an ascii code?

Answer 63

String.fromCharCode(68)

Question 64

How does array.find() work?

Answer 64

It returns the first value in the array that results to true to the provided callback function:

const array1 = [5, 12, 8, 130, 44];

const found = array1.find(element => element > 10);

console.log(found);
// expected output: 12

Question 65

true > null

Answer 65

// true – becomes 1 > 0

Question 66

What does this return?
‘a’.localeCompare(‘b’);

Answer 66

-1

Question 67

Under which conditions should you use the try, catch blocks?

Answer 67

1 - The built in function or method might throw an error and you need to handle it
2 - A simple guard cause is impossible or impractical

Question 68

Destructure the array but only 1 and 7
let bar = [1, 2, 3, 4, 5, 6, 7];
As if you were writing;
let first = bar[0];
let seven = bar[6];

Answer 68

let [ first, , , , , , seventh ] = bar;

Question 69

What does this return?
const str1 = ‘strunz’;
str1.startsWith(‘u’, 3);

Answer 69

true

Question 70

What does this return?
‘’ == false

Answer 70

true – becomes ‘’ == 0, then 0 == 0

Question 71

What is a type error?

Answer 71

When you call an inexistent method on an object or primitive.

Question 72

What does the code return?
[…Array(3)].fill(2)

Answer 72

[2, 2, 2]

Question 73

What is a reference error?

Answer 73

When you attempt to reference a variable or function that doesn’t exist

Question 74

How would a regex that would match either dog, cat or rat look like and what is the type of regex called?

Answer 74

(cat|dog|rat) it is called ‘alternation’

Question 75

Create a clone of an object with spread syntax

let foo = { qux: 1, baz: 2 };
// code here
console.log(bar); // { qux: 1, baz: 2 }
console.log(foo === bar); // false – bar is a new object

Answer 75

let bar = { …foo };

Question 76

123 <= ‘a’

Answer 76

// also false

Question 77

What does this return?
true == ‘1’

Answer 77

true – 1 == 1

Question 78

Describe four (4) ways to create a date object:

Answer 78

new Date();

new Date(value);

new Date(dateString);
// new Date(‘December 17, 1995 03:24:00’) or
// new Date(‘1995-12-17T03:24:00’)

new Date(year, month[, date[, hours[, minutes[, seconds[, milliseconds]]]]]);
// new Date(1976, 4, other optional args);

Passing in the date object directly also works (e.g., new Date(today)). However, this employs implicit coercion which we don’t recommend. Using the Date.prototype.getTime method makes it clear what is going on.

Question 79

123 > ‘a’

Answer 79

// false – ‘a’ is coerced to NaN; any comparison with NaN is false

Question 80

let foo = false;

Answer 80

undefined — statements always return undefined

Question 81

What is an expression?

Answer 81

any code that resolves to a value

Question 82

What is an element of an array?

Answer 82

a value who has a non negative integer as it’s key (property name)

Question 83

undefined === null

Answer 83

false

Question 84

How to get a value from an ascii code?

Answer 84

String.fromCharCode(68)

Question 85

What does the regex specify?
p{3, 7}

Answer 85

3 to 7 occurrences of p

Question 86

How do non strict equality operators == coerce the following:
‘23’ == 23

Answer 86

It coerce the string to be a number
23 === 23 => true

Question 87

What does the code do?

Array.from(‘foo’);

Answer 87

[“f”, “o”, “o”]

Question 88

How do you add two decimal points to a value?

Answer 88

value.toFixed(2);

Question 89

undefined == false

Answer 89

false

Question 90

What does the code log?

let guess = false ? 1 : 2;
console.log(guess);

Answer 90

2. Guess is false so it has the second argument passed to it

Question 91

What is the regex for the anchor at the end of a string?

Answer 91

$

Question 92

Use rest syntax to desctructure the following:

let foo = [1, 2, 3, 4];

console.log(bar); // 1
console.log(qux); // [2, 3, 4]

Answer 92

let [ bar, …qux ] = foo;

Question 93

What does the code log?

let guess = true ? 1 : 2;
console.log(guess);

Answer 93

1. Guess is true so the first argument is passed to it

Question 94

What does this return?
const str1 = ‘strunz’;
str1.endsWith(‘u’, 4);

Answer 94

true

Question 95

Name three changes strict code brings

Answer 95

Eliminates silent errors by throwing an exception

Prevents code than can inhibit javascript from optimising a program later on so that it runs faster

Prevents using names and syntax that may clash with future versions

Question 96

Use Array destructuring for this code:

let foo = [1, 2, 3];
let first = foo[0];
let second = foo[1];
let third = foo[2];

Answer 96

let [ first, second, third ] = foo;

Question 97

What does this return?

let friends = [‘Bob’, ‘Josie’, ‘Sam’];
let enemies = friends;
console.log(friends === enemies);

Answer 97

true

Question 98

What does this return?
‘fava’.charAt(2);

Answer 98

‘v’

Question 99

How does hoisting work differently depending on the type of declaration?

Answer 99

For a function declaration both the function name and function body get hoisted. For variables only the name does. var is initialised to undefined. let and const are said to be in a TEMPORAL DEAD ZONE until initialised and inaccessible until then

Question 100

What does this return? why?
‘123’ * 3

Answer 100

369
all arithmetic operators other than + convert operands to numbers

Question 101

What does this return?
‘0’ == false (two conversions)

Answer 101

true – becomes ‘0’ == 0, then 0 == 0

Question 102

What does this return?

let friends = [‘Bob’, ‘Josie’, ‘Sam’];
let enemies = [‘Bob’, ‘Josie’, ‘Sam’];
console.log(friends[0] === enemies[0]);

Answer 102

true

Question 103

What does this return?
null + true

Answer 103

1

Question 104

What is a pragma?

Answer 104

A language construct that tells the compiler to process the code in a different way. It’s not part of the language

Question 105

Closures are:

Answer 105

A lexical, rather than a runtime feature

Question 106

Which are the meta characters inside a character class?

Answer 106

^ \ - [ ]

Question 107

When is a property name an array index?

Answer 107

when it’s a non negative integer; (their values are called elements)
any other property name is not an array index and its associated value is not called element.
Array.prototype.indexOf returns -1 if the value it is passed is not an element of the array,

Question 108

Can i code
[1, 2] + [3]
and get [1, 2, 3] ?

Answer 108

no. you’ll get ‘1, 23’ use .concat() instead;
[1, 2].concat([3]) // [1, 2, 3]

Question 109

What does this return?
‘hello’.charAt(1);

Answer 109

“e”

Question 110

What is a closure?

Answer 110

a combination of a function and it’s lexical environment

Question 111

How do you prevent a Type Error from being thrown when a reduce() method encounters an empty array?
ex:
[].reduce(reducer);

Answer 111

[].reduce(reducer, 0)
the 0 provided will be the default value assigned to the accumulator

Question 112

What happens when the + operator has two operands and one is an object?
[1, 2] + 4

Answer 112

they are both converted to strings
‘1,24’

Question 113

How do you remove the first property from foo?
let foo = {
a: ‘hello’,
b: ‘world’,
};

Answer 113

delete foo.a;
foo; // { b: “world” }

Question 114

What is return value of statements?

Answer 114

The return value of statements in JavaScript is undefined, and the return value of an expression with an operator is operator dependent. For the assignment (=) operator, the return value is the value assigned to the variable on the left side.

Question 115

Does the closure include all the existing variables within the scope of the function?

Answer 115

no. only the ones that the function needs

Question 116

null <= false

Answer 116

// true – becomes 0 <= 0

Question 117

What’s the difference between String.substr() and String.substring() ?

Answer 117

The substr() method extracts parts of a string, beginning at the character at the specified position, and returns the specified number of characters. The substring() method returns the part of the string between the start and end indexes, or to the end of the string

Question 118

What is the technical name for …theArgs?
function sum(…theArgs) {
//…
}

Answer 118

Rest parameter

Question 119

What is the difference between String() and toString() ?

Answer 119

String() can convert all types, including null and undefined, while toString() raises an exception. Also String() always returns a string. Individual objects could override toString() though

Question 120

How do you use BigInt?

Answer 120

append an n to the number

Question 121

How to you convert a number, ex: 1234 into binary?

Answer 121

let num = 1234;
num.toString(2);

Question 122

What happens here?
[1] * 2;

Answer 122

The operation becomes ‘1’ * 2, then the ‘1’ is converted to 1. So we get 1 * 2 => 2

Question 123

What does this log?

const animals = [‘ant’, ‘bison’, ‘camel’, ‘duck’, ‘elephant’];

console.log(animals.splice(0));
console.log(animals);

Answer 123

[‘ant’, ‘bison’, ‘camel’, ‘duck’, ‘elephant’]; // the deleted items
[] // the mutated array

Question 124

What is an initializer?

Answer 124

the expression to the right of the = in a variable declaration

Question 125

What happens when the + operator has two operands and neither are strings or object?
true + false

Answer 125

they are converted into numbers
1 + 0 => 1

Question 126

What does this return?
5 + ‘41’

Answer 126

‘541’

Question 127

true > false

Answer 127

// true – also becomes 1 > 0

Question 128

How do you create an array from the ‘arguments’ pseudo array?

Answer 128

let args = Array.prototype.slice.call(arguments);

Question 129

Concatenate arrays with spread syntax

let foo = [1, 2, 3];
let bar = [4, 5, 6];
// code here
qux; // => [1, 2, 3, 4, 5, 6]

Answer 129

let qux = […foo, …bar];

Question 130

What does this return?
true == ‘true’

Answer 130

false – 1 == NaN

Question 131

Where should I not declare functions?

Answer 131

In non function blocks like “if else” statements. You should instead use arrow functions or function expressions

Question 132

NaN == 0

Answer 132

false

Question 133

What does it mean for javascript to be weakly typed?

Answer 133

You don’t have to use type specific variables to store values, like for instance in C: int i = 7

Question 134

What is the result of these comparisons:

let friends = [‘Bob’, ‘Josie’, ‘Sam’];
let enemies = [‘Bob’, ‘Josie’, ‘Sam’];
friends == enemies;
friends === enemies;
[] == [];
[] === [];

Answer 134

false
false
false
false

they are all different objects.
arrays are only equal when they are the same object.

Question 135

null == null

Answer 135

true

Question 136

What does this return?
42 == true

Answer 136

false – becomes 42 == 1

Question 137

What makes a pure function?

Answer 137

Have no side effects.

Given the same set of arguments, the function always returns the same value during the function’s lifetime. This rule implies that the return value of a pure function depends solely on its arguments.

Question 138

NaN === NaN

Answer 138

// false – even with the strict operator

Question 139

NaN == NaN

Answer 139

false

Question 140

What will happen with this code and why?

let counter = 5;
let rate = 3;

function counter(count) {
// …
}

console.log(‘The total value is ‘ + String(counter * rate));

Answer 140

It will raise an exception;
A function declaration creates a variable with the same name as the function name. If the variables had been declared with var it would have worked fine

Question 141

What does this return?
const str1 = ‘strunz’;
str1.startsWith(‘u’);

Answer 141

false

Question 142

What are higher order functions?

Answer 142

They are just normal functions with the extra ability to receive functions as arguments and also return them

Question 143

Why does the code result in an error?
let boxNumber = 356.toString(); // syntax error!

Answer 143

Javascript interprets a period after a number as a decimal point, not as a method separator. Do this instead: 356..toString(); or (356).toString();

Question 144

What does this log?

const animals = [‘ant’, ‘bison’, ‘camel’, ‘duck’, ‘elephant’];

console.log(animals.splice());
console.log(animals);

Answer 144

[] // the deleted items
[‘ant’, ‘bison’, ‘camel’, ‘duck’, ‘elephant’] // the ‘mutated’ array. not really mutated in this case

Question 145

What does the regex cover: \w

Answer 145

\w matches all letters, number and underscore

Question 146

null == false

Answer 146

false

Question 147

How does this work:
let state = ‘Mississippi’;

state.substring(6, 3);

Answer 147

// “sis”, starting at index 3 and ending at the one before index 6. The order of start or end doesn’t matter. The start is the smaller of the two. The last index isn’t included in the substring. The index at one less than the index supplied is

Question 148

What does the code return?
[…Array(3)].fill();

Answer 148

[undefined, undefined, undefined]

Question 149

What does the code log and why?

function myFunction() {
let a = 1;

if (true) {
console.log(a);
let a = 2;
console.log(a);
}
}

myFunction();

Answer 149

ReferenceError: Cannot access ‘a’ before initialization.

the let a = 2 inside the if block is hoisted but uninitialized. console.log(a) can’t access it and it also shadows the let a = 1

Question 150

undefined == undefined

Answer 150

true

Question 151

What is an array property name?

Answer 151

It’s a non negative index of an array. Values assigned to them are called elements

Question 152

Which are the primitive types?

Answer 152

Strings, numbers, booleans, undefined and null

Question 153

Can you use \b or \B word and non word boundaries in character classes?

Answer 153

No. \b means backspace in a character class

Question 154

What is the difference between slice() and substring()

Answer 154

slice() is for arrays while substring() if for strings.
they return a new array or string

Question 155

How and where do you enable strict mode?

Answer 155

“use strict”;

Can be used at the global or at the function level

Can’t be used inside a block. Top of the code or top of the function only

Question 156

Can you use arrow functions as methods?

Answer 156

Better not. They have subtle behaviour that makes them unsuitable. Will learn more later.
It is safe to use arrow functions in the body of a method; just don’t use them to define the actual method.

Question 157

How do you make “ “ three times bigger?

Answer 157

” “.repeat(3)

Question 158

((false && undefined) || ‘a’ || ‘’);

Answer 158

‘a’

Question 159

What does it mean for javascript to be dinamically typed?

Answer 159

The value of any variable can be changed for another value of a different type

Question 160

What does this return?
+‘1’

Answer 160

the number 1. Prepending a string with + turns it into a number if possible or NaN if not
+‘1a’ would return NaN

Question 161

Make a RegExp to include word boundaries

Answer 161

let regex = RegExp(\\b${uniqueString}\\b, ‘g’);

Question 162

How do non strict equality operators == coerce the following:
0 == ‘\n’

Answer 162

It coerce the string to be a number
0 === 0 => true

Question 163

How do you represent a non decimal character in regex?

Answer 163

\D

Question 164

What data type are object property names?

Answer 164

Strings. When we omit the quotes around a property name javascript converts it into a string anyways so that an entry like: true: ‘whatever’ is converted to ‘true’: ‘whatever’

Question 165

What does the code return?
Array(3)

Answer 165

[empty × 3]

Question 166

Return a random integer between 14 and 17

Answer 166

Math.floor(Math.random() * 4 + 14);

Question 167

What is this equal to?
[ …unused, …washed ]

Answer 167

unused.concat(washed)

Question 168

How else could you write array.join(‘’) ?

Answer 168

array.join``

Question 169

What does the code log?

console.log(Array.from(‘foo’))

Answer 169

Array [“f”, “o”, “o”]

Question 170

What does the code log?

console.log(Array.from([1, 2, 3], x => x + x))

Answer 170

Array [2, 4, 6]

Question 171

What does the code log?

Array.from(new Set(‘abc’));

Answer 171

[“a”, “b”, “c”]

Question 172

What does the code log?

[…‘abc’]

Answer 172

[“a”, “b”, “c”]

Question 173

What is encapsulation?

Answer 173

The grouping of related variables and functions that operate on them into objects.
Reduces the number of parameters the function takes.
Avoids functions being scattered throughout the program. This is problematic because interdependence between functions can lead to bugs or code possibly breaking if change to one function affects others

Question 174

What problem does OOP solve?

Answer 174

Reduces the interdependence of functions.
Sectioning off chunks of code has many benefits: reduced the impact of change to any one function, simplifies code, removes parameters from functions

Question 175

What is abstraction?

Answer 175

The hiding of properties and methods from outside the object. This has two benefits:
- It simplifies the object’s use and understanding
- It minimises the impact of change
if we decide to make changes to the private, or hidden, methods those changes won’t affect the code outside of the object

Question 176

What is polymorphism?

Answer 176

Two objects of different types can respond differently to the same method call.
Remove a bunch of if else statements

Question 177

How would you pass a function called, say, fullName(), to the following forEach() method?
function rollCall(people) {
// code here
}

Answer 177

people.forEach(fullName);

note that’s just a variable whose value is a function. We don’t add parenthesis to call the function.
Each element in the people array will be passed as an argument to the function: fullName(element);

Question 178

What happens when a function is invoked using the () syntax?

Answer 178

A context is assigned to the function.
Every function has a context assigned at execution time.

Question 179

What does the global window object do?

Answer 179

Provides variables and functions that are available throughout the program.

Question 180

What happens if you declare a global variable with var or a function?

Answer 180

They become a property of the window object. It doesn’t work with let and const. you have to use dot notation as with any other object:
window.example = ‘whatever’;

Question 181

What is the ‘this’ keyword?

Answer 181

It’s a synonym for the execution context. It’s the object that is housing the property.

Question 182

What does this return?
[1, 2, 3].splice(undefined, 1);

Answer 182

1
It removes the first element and mutates the array =>
[2, 3];

Question 183

What does this return?
[1, 2, 3].splice(-1, 1);

Answer 183

3
It removes the first element from the end and mutates the array =>
[1, 2];

Question 184

Is -1 truthy?

Answer 184

It is. All negative numbers are truthy.

Question 185

What is the value of b? Why?
let a = ‘fava’;
let b = a;
a = ‘banana’;
console.log(b);

Answer 185

‘fava’
With primitive values it’s not the reference that’s passed but the value itself. So when b is created to be equal to a there will be two ‘fava’ in memory. Each variable points to a different space in memory. The two items look the same but they are different things. This is an example of “pass by value”.

Question 186

What is stored on the stack?

Answer 186

Primitive values and references to objects. The objects references point to are stored in the heap. If there are several references to an object they will all point to the same object, which means that any one reference can mutate the object and other references will see the changes too.

Question 187

Video way to copy an object…

Answer 187

let a = {name: ‘dan’};
let b = Object.assign({}, a);
a.name = ‘fava’;
console.log(a.name); // fava
console.log(b.name); // dan

Question 188

What is logged by the code?

let a = 10;
let obj = {
a
}

let newObj = obj;
newObj.a += 10;

console.log(obj.a);
console.log(a);

Answer 188

20
10

Question 189

What is logged by the code?

let a = 10;
let obj = {
a
}

obj.a += 10;

console.log(obj.a);
console.log(a);

Answer 189

20
10

Question 190

What is logged by the code?

let a = 10;
let obj = {
a
}

let newObj = obj;
newObj.a += 10;

console.log(obj.a);
console.log(newObj.a);

Answer 190

20
20

Question 191

How would you call this type of function?

function makeCar(rate, braking) {
return {
speed: 0,
braking,
rate,
accelerate() {
this.speed += this.rate;
},
brake() {
this.speed -= this.braking;
if (this.speed < 0) {
this.speed = 0;
}
},
};
}

Answer 191

Object factory

Question 192

How does OOP work as opposed to procedural programming?

Answer 192

OOP uses objects as the building blocks of a program as opposed to local variables and functions.

Question 193

The object oriented code makes which questions easier to answer?

Answer 193

• What are the important concepts in the program?
• What are the properties of an object?
• How do we create an object?
• What operations can I perform on an object?
• Where should we add new properties and methods?

Question 194

What are the three main benefits of OOP?

Answer 194

• We organize related data and code together.
• It’s useful when a program needs more than one instance of something.
• It becomes more useful as the codebase size increases.

Question 195

What characterises a first class function?

Answer 195

• we can add them to objects
• execute them in the context of those objects
• remove them from their objects
• pass them to other functions and run them in entirely
  different contexts
• First-class Functions initially have no context; they
  receive one when the program executes them.

Question 196

What does javascript create when it starts running?

Answer 196

The global object. This is used as the implicit execution context. In the browser this is the window object

Question 197

What is the difference between declaring a variable with var and not declaring it, in the context of the global object?

Answer 197

You can delete variables that you don’t declare but not the ones you do!

If you declare it, you keep it!
Function declarations work the same as var variables

Question 198

Is the global object always window?

Answer 198

NO. Only in the browsers. In Node it’s ‘object’.
Also, Node introduces an additional “module” scope; variables declared in the outermost scope are placed in the module and not available in the global object. Undeclared variables, on the other hand, are!

Question 199

Are the this binding rules similar to closures and variable scope?

Answer 199

No. It’s not determined by the lexical code but by how the function is invoked

Question 200

What is another name for the implicit function execution context

Answer 200

Implicit binding for functions

Question 201

Does bind execute a function?

Answer 201

No. It creates a new one permanently bound to the specified execution context

Question 202

What will the code below log to console?

let obj = {
message: ‘JavaScript’,
};

function foo() {
console.log(this.message);
}

foo.bind(obj);

Answer 202

Nothing. Unlike call and apply, bind doesn’t invoke the receiver function. Rather, it returns a new function that is permanently bound to the context argument.

Question 203

What does this log?
let hobbies = {
a: ‘fencing’,
b: ‘mincing’,
foo() {
return this.a + ‘ and ‘ + this.b;
},
};

let bar = hobbies.foo();
console.log(bar);

Answer 203

fencing and mincing

Question 204

What does this log?

let hobbies = {
a: ‘fencing’,
b: ‘mincing’,
foo() {
return this.a + ‘ and ‘ + this.b;
},
};

let bar = hobbies.foo;
console.log(bar());

Answer 204

undefined and undefined

Question 205

What is logged here?

let obj = {
a: ‘hello’,
b: ‘world’,
foo() {
function bar() {
console.log(this.a + ‘ ‘ + this.b);
}

bar();   }, };

obj.foo();

Answer 205

undefined undefined

Even though foo executes within the obj context, the call to bar on line 9 does not provide an explicit context, which means that JavaScript binds the global object to the function. As a result, this on line 6 is the global object, not obj.

Question 206

Can you chain bind() to a function delcaration?

Answer 206

No. It must be a function expression

let bar = function() {
console.log(this.a + ‘ ‘ + this.b);
}.bind(this);

Chaining it to a function declaration would result in an error

Question 207

There are 4 workarounds to make sure the bar() function doesn’t lose it’s context. What are they?

let obj = {
a: ‘hello’,
b: ‘world’,
foo() {
function bar() {
console.log(this.a + ‘ ‘ + this.b);
}

bar();   }, };

obj.foo() // undefined undefined

Answer 207

1 -
let obj = {
a: ‘hello’,
b: ‘world’,
foo() {
function bar() {
console.log(this.a + ‘ ‘ + this.b);
}

bar.call(obj);   }, };

obj.foo()

2 - error here. Can’t add bind to function declarations
let obj = {
a: ‘hello’,
b: ‘world’,
foo() {
let self = this;

function bar() {
  console.log(self.a + ' ' + self.b);
}

bar();   }, };

obj.foo()

3 -
let obj = {
a: ‘hello’,
b: ‘world’,
foo() {
let bar = function() {
console.log(this.a + ‘ ‘ + this.b);
}.bind(obj);

bar();   }, };

obj.foo()

4 -
let obj = {
a: ‘hello’,
b: ‘world’,
foo() {
let bar = () => console.log(this.a + ‘ ‘ + this.b);

bar();   }, };

obj.foo()

Question 208

We can fix the below bug with the previously explored solutions. What is an easy additional way?

let obj = {
a: ‘hello’,
b: ‘world’,
foo() {
[1, 2, 3].forEach(function(number) {
console.log(String(number) + ‘ ‘ + this.a + ‘ ‘ + this.b);
});
},
};

obj.foo(); // undefined undefined

Answer 208

let obj = {
a: ‘hello’,
b: ‘world’,
foo() {
[1, 2, 3].forEach(function(number) {
console.log(String(number) + ‘ ‘ + this.a + ‘ ‘ + this.b);
}, this);
},
};

obj.foo();

We supply a second argument, this, to the forEach function. This argument is the context.

Array.prototype.map, Array.prototype.every, and Array.prototype.some also takes an optional thisArg argument.

Question 209

Do arrow functions have a this binding?

Answer 209

No. Arrow functions do not have a this binding. Instead of this being dependent on the location of the function invocation, JavaScript resolves it by looking at the enclosing scopes.

Question 210

Does a function passed in as an argument to another function keeps its execution context?

Answer 210

No. Inner functions lose the outer object as context.
It would as an arrow function, though.

Question 211

Define ‘this’

Answer 211

The current execution context of a function

Question 212

How does strict mode affect ‘this’ in function calls?

Answer 212

Without strict mode ‘this’ refers to the window object. In strict mode it’s undefined

Question 213

Remember…

Answer 213

All JavaScript code executes within a context. The top-level context in a web browser is the window object. All global methods and Objects (such as parseInt or Math) are properties of this object. In Node, the top-level context is called global. However, be aware that Node has some peculiarities that cause it to behave differently from browsers.

Question 214

The value of this changes based on …

Answer 214

…how you invoke a function, not how you define it.

Question 215

JavaScript has first-class functions which have the three following characteristics:

Answer 215

• You can add them to objects and execute them in the respective object’s contexts.
• You can remove them from their objects, pass them around, and execute them in entirely different contexts.
• They’re initially unbound, but dynamically bound to a context object at execution time.

Question 216

Does this method invocation use implicit or explicit execution context?

let obj = {
foo() {}
};

function bar() {}

obj.foo();

Answer 216

Implicit

Question 217

What does this log?

function helloFactory() {
return function() {
console.log(‘hi’);
};
}

console.log(helloFactory());

Answer 217

The return function:

function() {
console.log(‘hi’);
};

Question 218

How would you invoke the function so that the inner return function is executed immediately?

What does this log?

function helloFactory() {
return function() {
console.log(‘hi’);
};
}

console.log(helloFactory());

Answer 218

helloFactory()();

Question 219

What does it mean that a language has first-class functions?

Answer 219

It means that the language treats functions as values – that you can assign a function into a variable, pass it around etc.

First class functions are functions that are treated like an object (or are assignable to a variable)

Question 220

What’s the trade off of keeping data private?

Answer 220

Making data private can make it harder to extend the code.
You may not be able to add a function outside of the object to manipulate private data. You have to update the object itself by adding a method.

Question 221

When does the program garbage collect primitives and objects?

Answer 221

When the program has no more references to them

Question 222

In what way are function declarations different from function expressions?

Answer 222

As an expression it means that there is a value returned — the function

Question 223

Is garbage collection in JavaScript is responsible for memory deallocation?

Answer 223

Yes

Question 224

What will this log?

const person = {
firstName: ‘Rick ‘,
lastName: ‘Sanchez’,
fullName: this.firstName + this.lastName,
};

console.log(person.fullName);

Answer 224

NaN.
Anywhere outside a function, the keyword this is bound to the global object. If the keyword is used inside a function, then its value depends on how the function was invoked.

Question 225

What is the purpose of this code?
let args = Array.prototype.slice.call(arguments);

Answer 225

It generates a real array from the quasi array ‘arguments’.
Here we borrow the slice() method from the Array global object and with call(arguments) we give the ‘arguments’ context to the method. args will now be a real array whose elements are the elements in ‘arguments’;

Question 226

What does the object property __proto__ do?

Answer 226

It references Object prototype and it keeps looking up in the hierarchy of objects for a property it doesn’t find in the current.

Question 227

What are 2 disadvantages of factory functions?

Answer 227

• Every object created with the factory function has a full copy of all the methods, which can be redundant.
• There isn’t a way for us to inspect an object and know whether we created it from a factory function. This makes it difficult to identify whether an object is of a specific “type.”

Question 228

How do you crate a deep clone of
let arr = [1, 2, 3] ?

Answer 228

let fava = Array.prototype.slice.call(arr);
fava.pop();
console.log(fava); // [1, 2]
console.log(arr); // [1, 2, 3]

Question 229

Does the prototype property exist in common object?

Answer 229

No. It’s only found in functions. But the __proto__ property will point to the parent object:

let arr = [1, 2, 3];
let fava = Object.create(arr);
console.log(fava.__proto__); // [1, 2, 3]

Question 230

What does the new keyword do for us in
function ParentExample(name, points) {
// some code
}

let example = new Object.create(parentExample) ?

Answer 230

• It creates a new object and sets the __proto__ property in example to point to the prototype property of ParentExample, which itself refers to an object.
• It automatically returns the new object.

Question 231

What 4 things happen when we call a function with the new operator?

Answer 231

• A new object is created
• ‘this’ is set to the new object
• The function is run
• The new object is returned (or if not returned explicitly the ‘this’ keyword is returned instead.

Question 232

What is logged when inner() runs? Why?

let numbers = {

a: 1,

b: 2,

add() {
console.log(this.a + this.b); // 3
function inner() {
console.log(this.a + this.b);
}

inner();   } };

numbers.add()

Answer 232

Uncaught TypeError: Cannot read property ‘a’ of undefined.
The inner function does not inherit the ‘this’ from the add() method.

inner() is a function invocation, not a method. As such, it’s implicitly assigned the global object as execution context (or undefined in strict mode).

Question 233

Will assigning the myCat.logInfo method as an argument to the function work?

function Animal(type) {
this.type = type;

this.logInfo = function() {
console.log(this.type);
}
}
var myCat = new Animal(‘Cat’);
setTimeout(myCat.logInfo, 1000);

Answer 233

No.
setTimout(myCat.logInfo);
// is equivalent to:
var extractedLogInfo = myCat.logInfo;
setTimout(extractedLogInfo);

This is a solution:
setTimeout(myCat.logInfo.bind(myCat), 1000);

Question 234

Can a constructor be called without parenthesis if there are no arguments?

Answer 234

Yes, the parenthesis pair can be omitted: new FunctionName

Question 235

What is another way to write
this = Object.create(Fava.prototype);
?

Answer 235

Object.setPrototype(this, Fava.prototype);

Question 236

What are the 5 things that happen when we create an object with a constructor function using the new keyword?

Answer 236

• A new object is created
• The this keyword of the creator function is set to point to the new object
• The value of the function prototype property is the object that the new object inherits from
• It invokes the function
• It returns the new object unless another object is returned explicitly

Question 237

What objects implicitly get a prototype property in Javascript?

Answer 237

Every Function object will automatically get a prototype property which has an object as a value. This object will contain the constructor property. This will then point to the function itself

Question 238

What should methods defined on the prototype always do?

Answer 238

Return the context object!!

Question 239

What are three caveats about using classes?

Answer 239

All code in class executes in strict mode.

Unlike function declarations, class declarations are not hoisted.

Invoking the class constructor without the new keyword raises an error.

Question 240

What will this log?
console.log(typeof Object)

Answer 240

function! Objects, Arrays and Functions are functions, since each of their creator property points to Function. Function made them so they are instances of function

Question 241

What does this do?
Object.create(obj)

Answer 241

It creates a new object and sets it’s dunder proto to point to obj

Question 242

In node, where are variables declared with var or let stored?
var foo = ‘foo’

Answer 242

In the module scope

Question 243

In node, where are variables declared without stored?
foo = ‘foo’

Answer 243

In the global object.

Question 244

What happens to a function’s context when the function is used as an argument, like, say, a callback function?

Answer 244

The context is lost and it’s implicitly set to the global object. Undefined on strict mode. Unless they are arrow function that is.
The same happens to methods!! Passed in as arguments they lose their original binding

Question 245

What do the following all point to?
function fava() {}

Array.prototype.__proto__
Function.prototype.__proto__
fava.prototype.__proto__

Answer 245

Object.prototype

Question 246

What is ‘this’ in arrow functions?

Answer 246

‘this’ is the enclosing context where the arrow function is defined.

Question 247

Syntax errors occur during which phase?

Answer 247

Creation. As opposed to the execution phase.

let foo = “hello”;

function foo() {
console.log(“hello”);
}
At creation, when the engine reaches the function it’s aware that foo was already declared with let. So it raises an error

Question 248

const person = {
first: ‘R ‘,
last: ‘S’,
full: this.first + this.last,
};

console.log(person.full);

Answer 248

If you said it logs NaN, you’re correct. It is tempting to say that the code will log “R S” to the console but that’s not correct.

Anywhere outside a function, the keyword this is bound to the global object. If the keyword is used inside a function, then its value depends on how the function was invoked.

Question 249

Can you nest a function declaration within a non function block?

Answer 249

You shouldn’t but you can. Different browsers process them differently

Question 250

Is the underscore _ allowed in variable naming?

Answer 250

Yes but it’s not idiomatic

Question 251

Give 3 invalid variable naming styles

Answer 251

• Start variable with a number, ex. let 34example
• Hyphen anywhere, ex. let say-hi
• Dot anywhere, ex. say.hi

Question 252

What is an expression?

Answer 252

Any valid code that resolves to a value

Question 253

Where can we not use statements?

Answer 253

As part of an expression
let fava = (let lessa = 9)
the let lessa is a statement

Question 254

What does this log?
function foo() {
if (false) {
var a = 1;
}

console.log(a);
}

foo();

Answer 254

undefined

Question 255

What does this log

function bar() {
return ‘world’;
}

var bar;

console.log(bar());

bar = ‘hello’;

Answer 255

world

Question 256

What does this log

console.log(bar());

function bar() {
return ‘world’;
}

var bar = ‘hello’;

Answer 256

world

Question 257

what do you think happens if you run this code?

let foo = “hello”;

function foo() {
console.log(“hello”);
}

Answer 257

Syntax errors usually occur during the creation phase – before “hoisting” has an effect on the code. Since processing occurs from the top down during the creation phase, the first identifier found is the foo variable on line 1. When the creation phase reaches the function declaration on lines 3-5, JavaScript already knows about the foo identifier, so it complains that foo has already been declared. The error occurs on line 3, not line 1.

Question 258

Is there a function declaration in this code?

let foo = function () {
return function bar() {};
};

Answer 258

N0. A statement must begin with the function keyword to be a function declaration

Question 259

How can you deep clone an array?

let arr1 = [1, 2, 3, [1]];

Answer 259

let arr2 = JSON.parse(JSON.stringify(arr1));

arr1[3][0] = ‘a’;

console.log(arr1 === arr2); // false
console.log(arr1[3] === arr2[3]); // false

Question 260

What is JSON?

Answer 260

The JSON object contains methods for parsing JavaScript Object Notation

Question 261

What do we call these associations between a name (or key) and a value?

Answer 261

Properties

Question 262

The values represent the _____ of the object

Answer 262

attributes

Question 263

What does this do?
0 in []; // false
0 in [1]; // true

Answer 263

The in operator checks whether an Object contains a specific key.

Question 264

What does this log?

let fava = [];
fava[10] = ‘lessa’;
console.log(fava);
console.log(3 in fava);

Answer 264

[empty × 10, ‘lessa’]

false

Question 265

What does this return?
{} == {}

Answer 265

false
Like with arrays, two objects are considered equal by the == and === operators only if they are the same object:

Question 266

How do you see the property internal flags of an object?

let obj = {
a:1,
}

Answer 266

console.log(Object.getOwnPropertyDescriptors(obj));

{a: {…}}
a: {value: 1, writable: true, enumerable: true, configurable: true}
[[Prototype]]: Object

Question 267

How can we change the internal properties of an object?

Answer 267

let prices = [400, 80, 375, 870];
Object.defineProperty(prices, 1, { enumerable: false });

for (var k in prices) {
console.log(prices[k]);
}
// logs 400, 375, 870 … the 80 is missing!

However, even with enumerable flags set to false, other iteration methods such as the for…of loop or the built-in Array.prototype.forEach method will remain unchanged because they do not depend on this flag for instructions.

Question 268

What does this log?

var products = {
“widget”: 400,
“gear”: 80,
“crank”: 375,
“lever”: 870,
};

Object.defineProperty(products, “gear”, { enumerable: false });

var productKeys = Object.keys(products);
console.log(“productKeys: “, productKeys);

Answer 268

// logs productKeys: [ “widget”, “crank”, “lever” ]

Question 269

What is considered a side effect?

Answer 269

Anything that causes JavaScript to look outside the program for a place to read or send data is a side effect.

let date = new Date(); // side effect: accesses the system clock
let rand = Math.random(); // side effect: accessed random number generator

Question 270

How does a variable declared with ‘var’ behave when it’s declared inside a function?

Answer 270

It is not stored in the global object

Question 271

How does ‘continue’ work in a loop?

Answer 271

It skips the current iteration and goes back to loop over the next item

Question 272

How do you check if a value is NaN?

Answer 272

Number.isNaN(value)

Question 273

Are there any comparison with NaN which results to true?

Answer 273

No.
NaN === NaN
=> false

Question 274

How do null and undefined behaved when compared with other objects?

Answer 274

If the two values are either null or undefined it returns true.
If one is null or undefined and the other isn’t it returns false.

Question 275

How do non strict == equality operators behave in comparisons with different types if one is a number?

Answer 275

They convert the operands to numbers
true == 1
=> true

Question 276

When one of the operands is an object, ‘+’ will…
ex. [1, 2] + 3

Answer 276

convert the whole thing to a string
‘1,23’