Speaking JavaScript

Question 1

What is the effect of this instruction?
myFunction(y>=0 ? y: -y){ return y}

What about this one:
f = myFunction(y>0 ? y: -y);

Answer 1

In the first case, you will get an error (Unexpected token >=) because functions do not accept conditional statements as parameters;

In the second case, that is a function statement so it can take a conditional expression as a call argument;

Question 2

What is the effect of reading an unknown property?

Answer 2

A value of unknown property is undefined

Question 3

What is the value of a missing parameter?

Answer 3

Undefined

Question 4

What is the value of uninitialised variables?

Answer 4

Undefined

Question 5

What is the logical value of undefined and null?

Answer 5

Both are false

Question 6

What is the meaning of null?

Answer 6

It means “no object”. It is used where an object is expected

Question 7

What is the difference between typeof and instanceof?

Answer 7

typeof( ) is meant to be used with primitive values, whereas instanceof( ) is meant to be used with objects

Question 8

What is the value of: typeof(null)

Answer 8

It is ‘object’&raquo_space;> this is a Quirk

Question 9

What is the logical value of an Object?

Answer 9

Objects are considered as “true”

Question 10

What is the effect of: false && foo()

Answer 10

foo is never called

Question 11

What is the effect of : true || foo( )

Answer 11

foo is never called

Question 12

What is the logical value of: Infinity > NaN

Answer 12

false, because Infinity is bigger than any other number (except NaN)

Question 13

What is the effect of:
let s = “abc”;
s.len++;
console.log(s.len);

Answer 13

undefined.

Question 14

How do you enforce a function parity?

Answer 14

By checking the argument.length

Question 15

How do you convert arguments to an array?

Answer 15

Array.prototype.slice.call(myArguments);

Question 16

Can you remove elements from arguments?

Answer 16

No, because arguments is not an array

Question 17

What is “this” pointing to in an object’s method?

Answer 17

It’s pointing to the actual object

Question 18

What is the value of “this” inside a forEach method of an object’s property?

Answer 18

It is pointing to the global scope:
var jane = {
    name: 'jane',
    friends: 'Tarzan', 'Cheetah',
    yell: function( ) {
          this.friends.forEach(function(friend){
               console.log(this.name); // this point to global scope 
          })
    }
}

Question 19

What is “this” bound to in an object’s method? When is binding of “this” happening?

Answer 19

It is bound to that object (Crockford 1). The binding happens at invocation time.

Question 20

How do the object’s public methods get their context?

Answer 20

They get their context from the invocation of “this” in an object

Question 21

How is a function invoked when it is OT the property of an object?

Answer 21

It will be invoked as a function.

Question 22

What is “this” bound to in a function invocation pattern?

Answer 22

It is bound to the global object

Question 23

Can an object’s method employ an inner function to do any work? If yes, why, if not what is the solution?

Answer 23

the answer is no, because of the Crockford 2 “function invocation pattern”; the “this” in the inner function points to the global object. solutions: use “that” in the method, or use “bind”

Question 24

What is “this” bound to in a Constructor invocation pattern?

Answer 24

It is linked to the new Object (Crockford 3)

Question 25

What happens if you extract a method from an object in a separate function?

Answer 25

You will severe the “this” link to the object, so now “this” will point to the global scope

Question 26

What is “5 in myArray” instruction does?

Answer 26

it returns true if myArray has an element on position 5

Question 27

statements vs expressions&raquo_space;> what is the difference?

Answer 27

Expressions produce a value, statements perform an action

Question 28

What are object literals: expressions or statements?

Answer 28

Expressions

Question 29

What happens if you omit the outer parentheses below:

(function () { return ‘ac’}), and why?

Answer 29

You will get an error, as anonymous function declarations are not allowed

Question 30

What is “this” pointing to in a non-method function in “strict” mode?

Answer 30

undefined

Question 31

Why can you use “strict mode” to protect a constructor not be called without new?

Answer 31

Because if you don’t use “new” when calling the function, you actually call it in a non-method way, so if “strict” mode is active it be undefined

Question 32

What are the E6 primitive values?

Answer 32

boolean, numbers, strings, null, undefined, symbols

Question 33

What are the three characteristics of the primitive types? What are the three characteristics of objects?

Answer 33

Primitives:
1. They are always compared by value; (‘a’===’a’&raquo_space; true)
2. They are immutable (e.g. strings are immutable);
3. You cannot define your own set of primitive values
Objects:
1. They are compared by reference ({ } === { }&raquo_space;> false)
2. They are mutable by default;
3. They are user extensible;

Question 34

What is everything which is not a primitive in Javascript?

Answer 34

An object

Question 35

Enumerate different kinds of objects in Javascript

Answer 35

Plain objects (literals), Arrays,regexes

Question 36

What is the difference in terms of meaning for “undefined” and “null”? Give the use cases (when they appear) for both;

Answer 36

“undefined” means “no value” (neither primitive, nor object)&raquo_space;> uninitialized vars, missing params, missing properties; also functions return it if nothing is explicitely returned
“null” means “no object”&raquo_space;> it is used as a non-valued where an object is expected: a param, a member in a chain of objects

Question 37

Having the following function:
function returnFoo(x) { return x.foo}
What is the return value for the following:
1. returnFoo(true)
2. returnFoo(0)
3. returnFoo(null)
4. returnFoo(undefined)

Answer 37

1. undefined
2. undefined
3. TypeError: cannot read property “foo” of null
4. TypeError: cannot read property “foo” of undefined

Question 38

What is the last element in a prototype chain and how do you get it?

Answer 38

null

Object.getPrototypeOf(Object.prototype)

Question 39

How do you check for null and how do you check for undefined?

Answer 39

if (x === null)

if (x === undefined)

Question 40

How do you check for either null or undefined?

Answer 40

if (x!== undefined && x!==null) // does x have a value
if(x === undefined || x === null) // is x a non-value

Question 41

What are the javascript values that are considered “false”

Answer 41

false, 0, nan, ‘’

Question 42

What is 5+null and why?

Answer 42

5, because null coerced to a number is 0

Question 43

What is 5+undefined and why?

Answer 43

NaN, because undefined coerces to NaN

Question 44

What is the recommended method to converse 123 into a string?

Answer 44

String(123)

Question 45

What is the logical value of:

‘abc’ instanceOf String

Answer 45

false, as primitives are not equal to their wrappers

Question 46

What are the falsy values?

Answer 46

undefined, null
false
0, NaN
‘’

Question 47

What are considered as “truthy” values in javascript?

Answer 47

Everything which is not falsy

Question 48

What is the boolean value of an object?

Answer 48

truthy

Question 49

What is string(null)

Answer 49

‘null’

Question 50

What is String(false)

Answer 50

‘false’

Question 51

What do you get when you use:
let o = Object.create(null,{a:{value:1}}) and then use:
o.\_\_proto\_\_ ?

Answer 51

undefined