JavaScript data (primitives)

the building blocks of web apps

We can categorize basic forms of data, known as primitives, as follows:

• Number : a value we can calculate with another, e.g.

  • an integer like 3, -1 or 0

  • a floating point number like -273.15

• String : a string of alphanumeric characters

  • plain text like String or Hello world!

  • plain text that has non-calculatable numbers like Price: $200

• Boolean : a value that is either true or false

• null : an empty value

• undefined : a variable with nothing assigned (not even null)

• Object : a value with a special structure that organizes a set of properties and their values for a group of similar things

• Symbol : (beyond the scope of this introduction)

Example of null and undefined

let x = null
let y

console.log(x) 
// null

console.log(y) 
// undefined

Note that 0 does not mean the same thing as null in JavaScript:

• 0 represents a number with which we can do calculations

• null represents the intentional absence of a number

• undefined represents that we have not yet decided on a value or a null

typeof keyword

To see what the type of value a variable has, we use typeof before a variable or value:

let x = 3.1415;
console.log(typeof x)
// "number"

Copying by value

When we copy some primitives, we copy by value:

let x = 20
let y = x
y = 40
console.log(x)
// 20

Note that despite setting y equal to x, then changing y's value - the value of x does not change!

This happens with:

• a Number

• a String

• a Boolean

• a null

• an undefined

• a Symbol

An analogy of copying by value: copying an image from the internet and then modifying the copy in an image editing software; the original image does not change!

Copying by reference

However, with:

• an Object

• a function

• an Array

...we copy by reference:

let object1 = { 
    prop1: 100, 
    prop2: 'ax', 
    prop3: true 
}

let object2 = object1
object2.prop1 = 400

console.log(object1.prop1)
// 400

Notice how changing object2's property also changed object1's property automatically!

An analogy of copying by reference: editing a post on social media (our edit results in changing the original for all to see, as we change the original data!)