Data types

What you'll learn

The basic kinds of values in JavaScript and how to check which kind you have.

Why it matters

JavaScript treats a number and a piece of text differently. Knowing the type of a value explains why "5" + 5 is not 10.

Explanation

The everyday types are:

• number: 42, 3.14, -7.
• string: text in quotes, "hello" or 'hello'.
• boolean: only true or false.
• undefined: a variable that has no value yet.
• null: "nothing" on purpose.

The typeof operator tells you the type of a value.

A string in quotes is always text, even if it looks like a number.

Example

const age = 30;
const city = "Pune";
const isStudent = false;
let nothingYet;

console.log(typeof age);        // number
console.log(typeof city);       // string
console.log(typeof isStudent);  // boolean
console.log(typeof nothingYet); // undefined

console.log(5 + 5);             // 10
console.log("5" + 5);           // 55, text joined
console.log("5" * 2);           // 10, text turned into a number

Step-by-step walkthrough

1. Four variables are created, each with a different kind of value.
2. typeof prints number, string, boolean, then undefined because nothingYet was never given a value.
3. 5 + 5 is number plus number, so the answer is 10.
4. "5" + 5 has a string on one side, so + joins them into the text 55.
5. "5" * 2 has no meaning for text, so JavaScript converts "5" to the number 5 first and gives 10.

Try it yourself

Print the typeof of "42", 42 and true. Then try "10" - 3.

Common mistakes

• Adding a number and a numeric string. "5" + 5 is "55". Convert with Number("5") first.
• Writing True or False. JavaScript is case-sensitive: true and false.
• Confusing null and undefined. undefined means "never set", null means "set to nothing".
• Expecting typeof null to be null. It prints object, a long-standing quirk.

Quick quiz

Summary

• Main types: number, string, boolean, undefined, null.
• typeof reveals a value's type.
• + adds numbers but joins strings.

Next: making decisions with conditionals.