JavaScript Data Types

Every value in JavaScript has a type, such as text, a number or true/false.

The Data Types

The first seven are called primitive types. Objects (including arrays) can hold many values.

Strings

A string is text inside quotes. You can use double quotes, single quotes or backticks.

let shop = "Saddar Mobile Centre";
let owner = 'Bilal';
let note = `Open till 10 PM`;

Use one kind of quote inside the other: "It's open" works fine.

Numbers

JavaScript has one number type for both whole numbers and decimals.

let students = 35;
let petrolPrice = 272.89;
let big = 25e6;    // 25000000

Booleans

A boolean is either true or false. Comparisons create booleans.

let isOpen = true;
let passed = 45 >= 50;   // false

Undefined and Null

A variable with no value is undefined. null means you have set it to "nothing" on purpose.

let phone;          // undefined
let discount = null; // no discount for now

Checking the Type with typeof

<!DOCTYPE html>
<html>
<body>

<p id="demo"></p>

<script>
let results = [
  typeof "Ayesha",
  typeof 19,
  typeof true,
  typeof undefined,
  typeof { city: "Karachi" },
  typeof [10, 20]
];
document.getElementById("demo").innerHTML = results.join("<br>");
</script>

</body>
</html>

string
number
boolean
undefined
object
object

Notice that an array also shows "object". To check for an array, use Array.isArray(value).

Dynamic Types

In JavaScript, the same variable can hold different types at different times. This is called dynamic typing.

let x = 10;       // number
x = "ten";        // now a string
x = false;        // now a boolean
console.log(typeof x);

boolean

This is flexible but can cause mistakes. TypeScript adds fixed types on top of JavaScript to catch them.

Type Conversion

Values from input boxes are always strings. Convert them before doing maths:

let a = "20";
let b = "30";
console.log(a + b);                  // "2030"
console.log(Number(a) + Number(b));  // 50
console.log(String(99));             // "99"

Exercise