JS

Question 1

Escaping characters

Answer 1

Code	Output
\'	single quote
\"	double quote
\\	backslash
\n	newline
\r	carriage return
\t	tab
\b	word boundary
\f	form feed

Question 2

Length of a string

Answer 2

“some text”.length

Question 3

String values are…

Answer 3

Immutable. Which means that they cannot be altered once created. (this does not mean that myStr cannot be changed, just that the individual characters of a string literal cannot be changed)

Question 4

multi-dimensional array

Answer 4

[[some content],[other content]]

Question 5

Array values are…

Answer 5

Mutable. myArray[0] = 45;

Question 6

.push() does…

Answer 6

appends data to an array

Question 7

.pop() does…

Answer 7

pops and assigns the last element from an array to anything

Question 8

.shift() does…

Answer 8

removes the first element of an array and returns it

Question 9

.unshift() is…

Answer 9

like push but to the beginning of an array

Question 10

structure of the function

Answer 10

function functionName() {
console.log(something);
}

Question 11

Variables which are used without the var keyword…

Answer 11

…are automatically created in the global scope.

Question 12

Variables which are declared within a function, as well as the function parameters…

Answer 12

… have local scope

Question 13

It is ___ to have both local and global variables with the same name.

Answer 13

possible!

Question 14

When you have both local and global variables with the same name…

Answer 14

… the local variable takes precedence over the global variable.

Question 15

what is hoisting and what gets hoisted

Answer 15

adding vars and functions to memory when executing. they get added to the top of the file, so your function can be at the bottom but you can call it at the top

vars get partially hoisted (only var but not its value)

functions get fully hoisted

Question 16

Variables which are defined outside of a function block have

Answer 16

Global scope. This means, they can be seen everywhere in your JavaScript code.

Question 17

In JavaScript, scope refers to…

Answer 17

the visibility of variables

Question 18

Variables which are used without the var keyword are…

Answer 18

automatically created in the global scope. This can create unintended consequences elsewhere in your code or when running a function again. You should always declare your variables with var.

Question 19

It is possible to have both local and global variables with the same name. When you do this,

Answer 19

the local variable takes precedence over the global variable.

Question 20

You can use a _ statement to send a value back out of a function.

Answer 20

return

Question 21

A function can include the return statement but it does not have to. In the case that the function doesn’t have a return statement, when you call it,

Answer 21

the function processes the inner code but the returned value is undefined.

Question 22

=== vs ==

Answer 22

unlike the equality operator, which attempts to convert both values being compared to a common type, the strict equality operator does not perform a type conversion.

Question 23

determine the type of a variable or a value

Answer 23

typeof 3 // number

typeof ‘3’ // string

Question 24

using logical AND

Answer 24

&&

the same effect: nest if statements

Question 25

logical OR

Answer 25

||

returns true if either of the operands is true

Question 26

case statements for multiple if statements

Answer 26

switch (value){
case a:
   do something;
   break;
case b:
   do something;
   break;
}

Question 27

switch statements equivalent to final “else”

Answer 27

default:
do something;
break;

(should be the last case, just like else)

Question 28

If the break statement is omitted from a switch statement’s case,

Answer 28

the following case statement(s) are executed until a break is encountered.

Question 29

If you have multiple inputs with the same output, you can represent them in a switch statement like this:

Answer 29

switch(val) {
  case 1:
  case 2:
  case 3:
    result = "1, 2, or 3";
    break;
  case 4:
    result = "4 alone";
}

Question 30

When a return statement is reached,

Answer 30

the execution of the current function stops and control returns to the calling location.

Question 31

Objects are similar to arrays, except that instead of using indexes to access and modify their data,

Answer 31

you access the data in objects through what are called properties.

Question 32

In an object, you can also use numbers as properties. You can even omit the quotes for single-word string properties, as follows:

Answer 32

var anotherObject = {
  make: "Ford",
  5: "five",
  "model": "focus"
};

Question 33

if your object has any non-string properties

Answer 33

JavaScript will automatically typecast them as strings.

Question 34

There are two ways to access the properties of an object

Answer 34

dot notation (.) and bracket notation ([]), similar to an array.

Question 35

Dot notation is what you use

Answer 35

when you know the name of the property you’re trying to access ahead of time.

Question 36

Access object property through a variable

Answer 36

var playerNumber = 1;

myPlayer[playerNumber] // returns value for 1:

Question 37

add properties to an object

Answer 37

myDog.newProperty = someValue

Question 38

delete properties from an object

Answer 38

delete myDog.newProperty

Question 39

If you have tabular data, you can use an object to “lookup” values rather than a switch statement or an if/else chain.

Answer 39

This is most useful when you know that your input data is limited to a certain range.

Question 40

Testing Objects for Properties

Answer 40

We can use the .hasOwnProperty(propname) method of objects to determine if that object has the given property name. .hasOwnProperty() returns true or false if the property is found or not.

Question 41

The sub-properties of objects can be accessed by chaining together the dot or bracket notation.

Answer 41

ourStorage.cabinet[“top drawer”].folder2; // “secrets”

ourStorage.desk.drawer; // “stapler”

Question 42

Array bracket notation can be chained to access nested arrays.

Answer 42

ourPets[0].names[1]; // “Fluffy”

ourPets[1].names[0];

Question 43

while loop

Answer 43

while (condition) {
code
}

Question 44

for loop

Answer 44

for ([initialization]; [condition]; [final-expression])
e.g.
for (var i = 0; i < 5; i++) {
  ourArray.push(i);
}

Question 45

nested for loops

Answer 45

for (var i = 0; i < arr.length; i++){
    for (var j = 0; j < arr[i].length; j++){
do something
    }
}

Question 46

do…while

Answer 46

It is called a do…while loop because it will first do one pass of the code inside the loop no matter what, and then continue to run the loop while the specified condition evaluates to true.

var ourArray = [];
var i = 0;
do {
  ourArray.push(i);
  i++;
} while (i < 5);

Essentially, a do…while loop ensures that the code inside the loop will run at least once.

Question 47

Recursion is

Answer 47

the concept that a function can be expressed in terms of itself. Use the function within itself.

Question 48

parseInt()

Answer 48

parses a string and returns an integer

Question 49

Use the parseInt Function with a Radix

Answer 49

The parseInt() function parses a string and returns an integer. It takes a second argument for the radix, which specifies the base of the number in the string. The radix can be an integer between 2 and 36.

The function call looks like:

parseInt(string, radix);

Question 50

conditional operator, also called the ternary operator

Answer 50

can be used as a one line if-else expression.

The syntax is:

condition ? statement-if-true : statement-if-false;

Question 51

String multiple conditionals/ternary together

Answer 51

condition1 ? statement-if-true
: condition2 ? statement-if-true
: statement-if-false

Question 52

when using let

Answer 52

variable with the same name can only be declared once

Question 53

“use strict”

Answer 53

This enables Strict Mode, which catches common coding mistakes and “unsafe” actions.

Question 54

When you declare a variable with the let keyword inside a block, statement, or expression,

Answer 54

its scope is limited to that block, statement, or expression.

Question 55

const

Answer 55

has all the awesome features that let has, with the added bonus that variables declared using const are read-only. They are a constant value, which means that once a variable is assigned with const, it cannot be reassigned.

Question 56

A common practice when naming constants

Answer 56

use all uppercase letters, with words separated by an underscore

Question 57

objects (including arrays and functions) assigned to a variable using const

Answer 57

are still mutable:

const s = [5, 6, 7];
s = [1, 2, 3]; // throws error, trying to assign a const
s[2] = 45; // works just as it would with an array declared with var or let
console.log(s); // returns [5, 6, 45]

Question 58

To ensure your data doesn’t change

Answer 58

JavaScript provides a function Object.freeze to prevent data mutation.

Question 59

Arrow function syntax

Answer 59

const myFunc = () => “value”;

Question 60

How to display Default parameters in arrow functions

Answer 60

const myFunc = (arr1, arr2 = 1) => “value”;

Question 61

What does rest parameter do?

Answer 61

you can create functions that take a variable number of arguments. These arguments are stored in an array that can be accessed later from inside the function.

const myFunc = (...args) => {
return args.map }

Question 62

What does the spread operator allow?

Answer 62

allows us to expand arrays and other expressions in places where multiple parameters or elements are expected

const arr = [6, 89, 3, 45];
const maximus = Math.max(...arr)
...arr returns an unpacked array.

Question 63

Why use the Destructuring assignment?

Answer 63

for neatly assigning values taken directly from an object.

var user = {name: "John", age: '34'};
const { name, age } = user;
// name = 'John Doe', age = 34
-- this picks up the keys and matches them and returns their values, so even if there were more keys in the obj, there would be no need to specify them

Question 64

One key difference between the spread operator and array restructuring is that…

Answer 64

the spread operator unpacks all contents of an array into a comma-separated list. Consequently, you cannot pick or choose which elements you want to assign to variables.

Destructuring an array lets us do exactly that:

const [a, b] = [1, 2, 3, 4, 5, 6];
console.log(a, b); // 1, 2
The variable a is assigned the first value of the array, and b is assigned the second value of the array

Question 65

We can also access the value at any index in an array with destructuring

Answer 65

by using commas to reach the desired index:

const [a, b,,, c] = [1, 2, 3, 4, 5, 6];
console.log(a, b, c); // 1, 2, 5

Question 66

involving array destructuring, we might want to collect the rest of the elements into a separate array.

Answer 66

const [a, b, …arr] = [1, 2, 3, 4, 5, 7];

console. log(a, b); // 1, 2
console. log(arr); // [3, 4, 5, 7]

Question 67

Send stuff into a function from an object

Answer 67

const stats = {
  max: 56.78,
  standard_deviation: 4.34,
  median: 34.54,
  mode: 23.87,
  min: -0.75,
  average: 35.85
};

const half = ({max,min}) => (max + min) / 2.0;

Question 68

template literal (what it allows, how to use it)

Answer 68

Template literals allow you to create multi-line strings and to use string interpolation features to create strings.

const person = {
name: “Zodiac Hasbro”,
age: 56
};

// Template literal with multi-line and string interpolation
const greeting = `Hello, my name is ${person.name}!
I am ${person.age} years old.`;

console.log(greeting); // prints
// Hello, my name is Zodiac Hasbro!
// I am 56 years old.

Question 69

template literal allows you to…

Answer 69

create multi-line strings and to use string interpolation features to create strings.

Question 70

Write Concise Object Literal Declarations Using Object Property Shorthand

Answer 70

ES6 provides the syntactic sugar to eliminate the redundancy of having to write x: x. You can simply write x once, and it will be converted tox: x (or something equivalent) under the hood.

Instead of this
const getMousePosition = (x, y) => ({
  x: x,
  y: y
});

you do this
const getMousePosition = (x, y) => ({ x, y });

Question 71

Write Concise Object Literal Declarations Using Object Property Shorthand

Answer 71

ES6 provides the syntactic sugar to eliminate the redundancy of having to write x: x. You can simply write x once, and it will be converted tox: x (or something equivalent) under the hood.

Instead of this
const getMousePosition = (x, y) => ({
  x: x,
  y: y
});

you do this
const getMousePosition = (x, y) => ({ x, y });

Question 72

When defining functions within objects in ES5, we have to use the keyword function as follows

Answer 72

const person = {
  name: "Taylor",
  sayHello: function() {
    return `Hello! My name is ${this.name}.`;
  }
};

Question 73

With ES6, You can remove the function keyword and colon altogether when defining functions in objects.

Answer 73

const person = {
  name: "Taylor",
  sayHello() {
    return `Hello! My name is ${this.name}.`;
  }
};

Question 74

The class syntax in ES6 simply replaces the constructor function creation:

Answer 74

class SpaceShuttle {
  constructor(targetPlanet) {
    this.targetPlanet = targetPlanet;
  }
}
const zeus = new SpaceShuttle('Jupiter');

Question 75

The class syntax in ES6 simply replaces the constructor function creation:

Answer 75

class SpaceShuttle {
  constructor(targetPlanet) {
    this.targetPlanet = targetPlanet;
  }
}
const zeus = new SpaceShuttle('Jupiter');

Question 76

You can obtain values from an object and set the value of a property within an object. What are they called?

Answer 76

These are classically called getters and setters.

Question 77

Getter functions are meant to

Answer 77

simply return (get) the value of an object’s private variable to the user without the user directly accessing the private variable.

Question 78

Setter functions are meant to

Answer 78

modify (set) the value of an object’s private variable based on the value passed into the setter function. This change could involve calculations, or even overwriting the previous value completely.

Question 79

How to use getter and setter in a class

Answer 79

class Book {
  constructor(author) {
    this._author = author;
  }
  // getter
  get writer() {
    return this._author;
  }
  // setter
  set writer(updatedAuthor) {
    this._author = updatedAuthor;
  }
}
const lol = new Book('anonymous');
console.log(lol.writer);  // anonymous
lol.writer = 'wut';
console.log(lol.writer);  // wut

Question 80

How to use getter and setter in a class

Answer 80

class Book {
  constructor(author) {
    this._author = author;
  }
  // getter
  get writer() {
    return this._author;
  }
  // setter
  set writer(updatedAuthor) {
    this._author = updatedAuthor;
  }
}
const lol = new Book('anonymous');
console.log(lol.writer);  // anonymous
lol.writer = 'wut';
console.log(lol.writer);  // wut

Question 81

Two components of tag

Answer 81

type, src

Question 82

Export a function from a module

Answer 82

//defined function
const add = (x, y) => {
  return x + y;
}

//export
export { add };

Question 83

importing a function from a module

Answer 83

import { add } from ‘./math_functions.js’;

Question 84

import everything from a module

Answer 84

import * as myMathModule from “./math_functions.js”;

Question 85

why use export default

Answer 85

Usually you will use this syntax if only one value is being exported from a file.

It is also used to create a fallback value for a file or module.

Question 86

examples using export default

Answer 86

// named function
export default function add(x, y) {
  return x + y;
}

// anonymous function
export default function(x, y) {
  return x + y;
}

Question 87

you can only have ___ value be a default export in each module or file

Answer 87

one value

Question 88

you ____ use export default with var, let, or const

Answer 88

cannot

Question 89

importing a default

Answer 89

import add from “./math_functions.js”;

The syntax differs in one key place. The imported value, add, is not surrounded by curly braces ({}).

Question 90

You _____ use any name here when importing a default.

Answer 90

can

Question 91

Promise is a ___ function, so you need to use the ___ keyword to create one.

Answer 91

constructor

new

Question 92

Promise takes a function, as its argument, with two parameters - ___ & _____. These are methods used to determine the outcome of the promise.

Answer 92

resolve, reject

Question 93

Promise syntax looks like this:

Answer 93

const myPromise = new Promise((resolve, reject) => {

});

Question 94

A promise has three states:

Answer 94

pending, fulfilled and rejected

Question 95

resolve (promise) is used

Answer 95

when you want your promise to succeed

Question 96

reject (promise) is used

Answer 96

when you want the promise to fail

Question 97

using promise and reject in the promise body

Answer 97

const makeServerRequest = new Promise((resolve, reject) => {
  // responseFromServer represents a response from a server
  let responseFromServer;

  if(responseFromServer) {
    resolve("We got the data");
  } else {  
    reject("Data not received");
  }
});

Question 98

Promises are most useful when

Answer 98

you have a process that takes an unknown amount of time in your code (i.e. something asynchronous), often a server request.

Question 99

The then method is executed ____after your promise is fulfilled with resolve.

Answer 99

immediately

Question 100

example code of a then in promise

Answer 100

myPromise.then(result => {
  // do something with the result.
});

//result comes from the argument given to the resolve method

Question 101

catch is the method used when

Answer 101

your promise has been rejected. executed, just like then, immediately after reject is called

Question 102

syntax for catch in promise

Answer 102

myPromise.catch(error => {
  // do something with the error.
});

//error is the argument passed in to the reject method

Question 103

Arrow functions allow us to…

Answer 103

write shorter function syntax

instead of

hello = function() {
return “Hello World!”;
}

you write

hello = () => {
return “Hello World!”;
}

Question 104

If the arrow function has only one statement, and the statement returns a value…

Answer 104

you can remove the brackets and the return keyword:

hello = () => “Hello World!”;