Check a function is defined in JavaScript

In JavaScript, checking if a function is defined helps prevent runtime errors before calling it. This ensures the code runs safely even if a function might not exist in the current scope.

• Use typeof functionName === "function" to verify if a function exists.
• Prevents “undefined is not a function” errors.
• Works for both user-defined and built-in functions.
• Useful in dynamic or modular code where functions may load conditionally.

JavaScript typeof Operator

The typeof operator is used to find the type of a JavaScript variable. This operator returns the type of a variable or an expression.

• Parameters: It contains a single parameter var which is a JavaScript variable.
• Return value: It returns the type of a variable or an expression.

Examples to Check a Function is Defined or Not in JavaScript

let defined = 'Not defined';

if (typeof fun === 'function') {
    defined = "Defined";
}

console.log("Function " + defined);

Function Not defined

Syntax:

typeof var

Example 2: This example checks the type of the function, If it is a function then it is defined otherwise not defined by using typeof operator by creating a function.

function isFunction(possibleFunction) {
    return typeof possibleFunction === 'function';
}

function fun() { }

let defined = 'Not defined';

if (isFunction(fun)) {
    defined = "Defined";
}

console.log("Function " + defined);

Function Defined