General Primitive Knowledge

Question 1

typeof 23.5;

typeof ‘Call me Ishmael.’;

typeof false;

typeof 0;

typeof [ ]

typeof { }

typeof null;

typeof undefined;

typeof Infinity

Answer 1

number

string

boolean

number

object

object

object

undefined

number

Question 2

Why does typeof null return “object”

Answer 2

Basically a legacy mistake.

In the first implementation of JavaScript, JavaScript values were represented as a type tag and a value.

The type tag for objects was 0.nullwas represented as the NULL pointer (0x00 in most platforms).

Consequently, null had 0 as type tag, hence the “object”typeofreturn value.

Question 3

Name all the data types

How many are there?

Answer 3

8

Number
Boolean
String

Symbol
BigInt

null
undefined

Object

Question 4

What are the data type categories?

How do the data types fit into them?

Answer 4

Primitive and composite
Primitive data types
1 Number
2 String
3 Boolean

4Symbol
5 BigInt

6 Null
7 undefined

Everything else is an object or Composite data type
8 Object
Array
Functions

Question 5

What makes a composite data type?

What makes a primitive?

Answer 5

Composite typesare made up of heterogeneous data as one unit.

A composite type can contain not only strings, numbers, Booleans,undefinedvalues, and null values, but even other composite types.

JavaScript supports three composite types: objects, arrays, and functions.

A primitive in JS is defined as NOT an object

When a primitive value is assigned to a variable (eg let foo = ‘bar’), the variable is set to that value directly.
An object variable value is set to the destination in memory where the object data is stored.

Question 6

Explicitly change data

• return argument as a number (2 main ways)?
• return argument as a string, 2 ways
• return an integer if it’s in the argument
• return a float if it’s in the argument

Answer 6

Number(‘424’), +’1’

String(4124),
Object.prototype.toString(number base)
ex (12312).toString(10);

Number.parseInt( )
Number.parseFloat( )
parseInt and parseFloat find the first number possible:Number.parseFloat(‘11.11fda2 1’ //returns 11.11

But they will return NaN if a non number comes first::Number.parseFloat(‘a11.11’) //returns NaN

Question 7

Where are primitive and reference values stored?

Answer 7

Primitive values are data that are stored on the stack. Primitive value is stored directly in the location that the variable accesses.

Reference values are objects that are stored in the heap.

Question 8

Can you change a primitive?

Answer 8

No, they can be created and destroyed but not changed like objects

(which means there are no mutating methods for strings, numbers, and boolean.

Question 9

Check if something is specifically NaN

Answer 9

Number.isNaN()

or

let isNaN = function(x) {

return !(x===x);

}

(NaN is the only thing that is not equivalent with itself)

Question 10

Write a function that checks whether the input is specifically NaN

Answer 10

NaN is the only thing that does not equal itself function isItNaN(input) { if (input !== input) { console.log(true); } else { console.log(false); } }

Question 11

What happens:

let b = (a = 1);

console. log(b);
console. log(a);

Why? (Incomplete)

Answer 11

evaluates to:

1

1

The a variable is declared n.

Question 12

Why doesn’t NaN === NaN evaluate to true?

Answer 12

NaN is designed to propagate through all calculations, infecting them like a virus, so if somewhere in your deep, complex calculations you hit upon a NaN, you don’t bubble out a seemingly sensible answer.

Question 13

null == undefined

Answer 13

true

but null === undefined evaluates to false

Question 14

When using > < >= and <=

What happens with null and undefined?

Answer 14

they are converted to numbers

(same as +null or Number(null) etc.)

(null to 0, undefined to NaN)

Question 15

alert( undefined > 0 );
alert( undefined < 0 );
alert( undefined == 0 );

Answer 15

// false (1)

// false (2)

// false (3)

• undefined is converted to NaN

Question 16

How are null and undefined different from other primitives

Answer 16

Null and undefined, have no wrapper type like strings, booleans and numbers do. If you try t o assign a value to them it will throw an error.

You can with NaN. Like other primitives, it won’t stick but it won’t throw an error.

Question 17

How do you assign a property to a primitive?

Answer 17

You can’t (or at least we don’t know how to yet)
Primitive types behave like objects when you use the property access notations, but you can’t assign new properties to them. Primitives get wrapped with an object temporarily, and then that object is immediately thrown away. Any attempt to assign values to properties will seem to succeed (even Nan), but subsequent attempts to access that new property will fail.

You can, however, assign them to String, Object, and Number. Because they are proper objects. They don’t just receive a wrapper like primitives.

Question 18

Are undefined, null and NaN primitives?

What does typeof return for undefined, null and NaN

Answer 18

Undefined, Null and NaN are primitive.

typeof null will return object, but it is a primitive

typeof NaN will return number

typeof undefined will return undefined

Question 19

‘beta’ < ‘gamma’

Answer 19

true

Question 20

[1,2] < [6, 7]

[12,2] < [6]

[72,2] < [6]

Answer 20

true

true

false

Compares them like strings

Question 21

{a: ‘a’} < {z: ‘z’}

{a: ‘a’} > {z: ‘z’}

Answer 21

false

false