JavaScript Typeof Operator

The typeof operator in JavaScript is used to determine the type of a given operand. It returns a string indicating the type of the operand’s value.

Syntax

typeof operand

• operand: The value or expression whose type you want to determine.

Types Returned by typeof

The typeof operator returns one of the following strings:

• "undefined": If the value is undefined.
• "object": If the value is null or an object (arrays, dates, objects, etc.).
• "boolean": If the value is a boolean (true or false).
• "number": If the value is a number (including NaN).
• "bigint": If the value is a BigInt.
• "string": If the value is a string.
• "symbol": If the value is a symbol.
• "function": If the value is a function.

Examples

1. Checking Basic Data Types:

console.log(typeof 42);           // Output: "number"
console.log(typeof 'hello');      // Output: "string"
console.log(typeof true);         // Output: "boolean"
console.log(typeof undefined);    // Output: "undefined"
console.log(typeof Symbol());     // Output: "symbol"
console.log(typeof BigInt(123));  // Output: "bigint"
console.log(typeof null);         // Output: "object" (historical quirk)
console.log(typeof {});           // Output: "object"
console.log(typeof []);           // Output: "object"
console.log(typeof function(){}); // Output: "function"

2. Checking Variables:

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

x = 42;
console.log(typeof x); // Output: "number"

x = 'hello';
console.log(typeof x); // Output: "string"

x = null;
console.log(typeof x); // Output: "object"

3. Using typeof in Conditional Statements:

function checkType(value) {
    if (typeof value === 'string') {
        console.log('It is a string!');
    } else if (typeof value === 'number') {
        console.log('It is a number!');
    } else {
        console.log('It is some other type!');
    }
}

checkType('hello'); // Output: It is a string!
checkType(42);      // Output: It is a number!
checkType(true);    // Output: It is some other type!

Special Cases

1. null Returns "object":
   • This is a known issue in JavaScript for historical reasons. Use strict comparison to check for null.

     let x = null;
     console.log(typeof x); // Output: "object"
     console.log(x === null); // Output: true
     

     Try It Now

2. NaN is of Type "number":
   • Although NaN stands for “Not-A-Number”, its type is "number".

     console.log(typeof NaN); // Output: "number"
     

     Try It Now

Conclusion

The typeof operator is a handy tool for identifying the type of a variable or value in JavaScript.