JavaScript typeof

JavaScript typeof Operator: A Complete Guide

JavaScript is a dynamically-typed language, which means that the data types of variables are determined automatically at runtime. However, sometimes it can be useful to know the data type of a particular value or variable. This is where the typeof operator comes in.

In this article, we’ll take a comprehensive look at the typeof operator in JavaScript, including what it is, how it works, and some practical examples of how it can be used.

What is the `typeof` operator?

The typeof operator is a built-in JavaScript operator that returns a string indicating the type of the operand. It takes a single argument and returns a string representing the data type of that argument. The syntax for using typeof is as follows:

typeof operand

The operand can be any expression or variable name. The return value is a string that can be one of the following:

"undefined": If the operand is undefined.
"boolean": If the operand is a Boolean value (true or false).
"number": If the operand is a number.
"bigint": If the operand is a BigInt.
"string": If the operand is a string.
"symbol": If the operand is a symbol.
"function": If the operand is a function.
"object": If the operand is an object (including arrays, null, and objects created with Object() or new Object()).
"null": If the operand is null.

Let’s look at some examples to understand the typeof operator better.

Examples

Example 1: `typeof` with primitive types

typeof undefined; // "undefined"
typeof true; // "boolean"
typeof 42; // "number"
typeof "hello"; // "string"
typeof Symbol(); // "symbol"

Example 2: `typeof` with functions

typeof function(){}; // "function"
typeof class {}; // "function"

Example 3: `typeof` with objects

typeof {}; // "object"
typeof []; // "object"
typeof null; // "object"
typeof new Object(); // "object"

In the third example above, it may be surprising that typeof null returns "object". This is because null is actually an empty object reference.

Common Use Cases

The typeof operator is a useful tool in a number of scenarios. Here are some common use cases:

1. Checking if a variable is defined

if (typeof someVariable !== "undefined") {
  // someVariable is defined
} else {
  // someVariable is undefined
}

2. Checking the data type of a variable

if (typeof someVariable === "number") {
  // someVariable is a number
}

3. Handling null values

function doSomething(arg) {
  if (typeof arg === "object" && arg !== null) {
    // arg is an object (excluding null)
  }
}

4. Checking if a value is a function

if (typeof someValue === "function") {
  // someValue is a function
}

The typeof operator is a powerful tool in JavaScript that can help you determine the data type of a variable or value. It’s especially useful when working with dynamic data types or when handling values that may be null or undefined.

What is the typeof operator?