What does typeof operator do?

JavaScript

typeof operator

The typeof operator in JavaScript returns a string indicating the data type of an operand. Let me break down how it works with different types:

javascript

// Numbers (regular and NaN)
typeof 42; // "number"
typeof 3.14; // "number"
typeof NaN; // "number"

// Strings
typeof 'hello'; // "string"
typeof ''; // "string"

// Booleans
typeof true; // "boolean"
typeof false; // "boolean"

// Undefined
typeof undefined; // "undefined"

// Objects and null
typeof {}; // "object"
typeof null; // "object" (this is a known JavaScript quirk)
typeof []; // "object" (arrays are objects in JavaScript)

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

// Symbol (added in ES6)
typeof Symbol(); // "symbol"

// BigInt (added in ES2020)
typeof 42n; // "bigint"

There are a few important gotchas to be aware of:

typeof null returns "object", which is a historical bug in JavaScript
Arrays are considered objects, so typeof [] returns "object"
You can use typeof on undeclared variables without causing an error, it will return "undefined"

The operator is particularly useful for type checking and defensive programming, like checking if a variable exists before using it:

javascript

if (typeof someVariable !== 'undefined') {
  // safe to use someVariable
}

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What does typeof operator do?

What does typeof operator do?

typeof operator

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