? JavaScript `typeof` Operator

⚡ Quick Overview

The typeof operator in JavaScript is used to find out the type of a value, variable, or expression. It always returns the type as a string, such as "number", "string", or "boolean".

? Key Concepts

typeof checks the type of a value at runtime.
It returns a string that names the type (for example, "string" or "object").
Works with variables, literals, and expressions.
Some values have special behaviors, like null and arrays.
Useful in debugging, validation, and generic functions that accept multiple types.

? typeof Return Values

The typeof operator can return one of these strings:

"undefined"
"object"
"boolean"
"number"
"string"
"symbol"
"function"
"bigint"

? Syntax

There are two common ways to use typeof:

? View Syntax

typeof variableName
typeof (expression)

You can use typeof directly with a variable or wrap an expression in parentheses. In both cases, the result is a string.

? Basic Examples

? View Basic typeof Examples

console.log(typeof "Hello");          // "string"
console.log(typeof 42);               // "number"
console.log(typeof true);             // "boolean"
console.log(typeof undefined);        // "undefined"
console.log(typeof { name: "John" }); // "object"
console.log(typeof [1, 2, 3]);        // "object" (arrays are objects)
console.log(typeof null);             // "object" (historical quirk)
console.log(typeof function(){});     // "function"

? typeof with Changing Values

Here is how typeof behaves when the same variable changes type:

? View Type Change Example

let x;
console.log(typeof x);     // "undefined"

x = 100;
console.log(typeof x);     // "number"

x = "JavaScript";
console.log(typeof x);     // "string"

x = true;
console.log(typeof x);     // "boolean"

x = null;
console.log(typeof x);     // "object" (special case)

? typeof with Arrays

Arrays return "object" with typeof. To be sure a value is an array, use Array.isArray().

? View Array Type Check Example

let arr = [1, 2, 3];

console.log(typeof arr);          // "object"
console.log(Array.isArray(arr));  // true

? Live Output / Explanation

? How to Read the Console Results

typeof "Hello" → returns "string" because the value is text.
typeof 42 → returns "number" for any numeric value (integer or decimal).
typeof true → returns "boolean" for true or false.
typeof undefined → returns "undefined" when a variable has no value.
typeof { name: "John" } → returns "object" for plain objects.
typeof [1, 2, 3] → also returns "object", because arrays are a kind of object.
typeof null → returns "object" due to a historical bug in JavaScript.
typeof function() {} → returns "function" for functions.

? When to Use typeof

To check if a variable was initialized or is still undefined.
To write functions that handle different input types safely.
To debug unexpected values in your code.
To distinguish between primitives like "string", "number", and "boolean".

? Tips & Best Practices

typeof is great for quick debugging and runtime type checking.
To check for null, use a strict comparison: x === null.
To check if a value is an array, use Array.isArray(x) instead of typeof.
Remember that typeof always returns a string like "number", not the keyword number.
Use typeof carefully with objects; many different structures (arrays, dates, objects) all return "object".

? Try It Yourself

Check the type of a BigInt value, for example: typeof 123n.
Create a function and check its type with typeof myFunction.
Check the type of null and undefined in the console.
Declare a variable without assigning a value and see what typeof returns.
Use Array.isArray() along with typeof to compare the results for arrays.
Open the browser console and try typeof on different values you use in your own code.