Type inference

Learn JavaScript's dynamic type inference.

Type inference

JavaScript is a dynamically typed language: you do not declare a variable's type explicitly; the engine infers it from the value.

For example, when you write let count = 42, count becomes a number. When you write let name = 'Alex', name becomes a string. You can later reassign the same variable with a value of a different type.

To check the type, you use the typeof operator; we will see it in detail in the next group.

Type checking with typeof

The typeof operator returns the type of a value as a string. For example, typeof 42 returns 'number'; typeof 'hello' returns 'string'.

This is useful when you want to behave differently based on a variable's type. For example, you can check whether an argument passed to a function is a number or a string.

Interestingly, typeof null returns 'object'; this is an old design decision in JavaScript.

Exercise Print types with typeof

Declare let count = 42 and let name = 'Alex'. Then call console.log(typeof count) and console.log(typeof name) to print their types.

Show example solution

let count = 42;
let name = "Alex";
console.log(typeof count);
console.log(typeof name);

Test yourself

Dynamic type

let x = 5; x = ____ // x is now a string

Show answer

let x = 5; x = 'text'; // x is now a string

Match the type concepts

typeof 42

number
typeof 'text'

string
typeof true

boolean
typeof undefined

undefined
How does type inference work in JavaScript?
- The type must be declared explicitly
- The variable's type is determined at compile time
- The variable's type is determined at runtime
- The type can never change
Show answer

The variable's type is determined at runtime — JavaScript is dynamically typed, so the type is determined at runtime.

Next lesson: Destructuring

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