Check a function is defined in JavaScript

In JavaScript, you can check whether a function is defined before calling it. This helps prevent runtime errors when a function may not exist in the current scope.

• Use the typeof operator to check whether a value is a function.
• typeof functionName === "function" returns true when the function is defined.
• It can be used with user-defined and built-in functions.
• It is useful when functions may be loaded conditionally or dynamically.

Using the typeof Operator

The typeof operator returns a string indicating the type of a value. Since JavaScript returns "function" for functions, it can be used to determine whether a function is defined.

Syntax:

typeof variable

Working:

• Use typeof with the function name.
• Compare the returned value with "function".
• If the result is "function", the function is defined and can be called.
• Otherwise, the function is not defined.

Example 1: Checking an Undefined Function

let defined = "Not defined";

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

console.log("Function " + defined);

Function Not defined

Here, fun is not defined, so typeof fun returns "undefined" instead of throwing a ReferenceError.

Example 2: Checking a Defined Function

function fun() {
    console.log("Function called");
}

let defined = "Not defined";

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

console.log("Function " + defined);

Function Defined

Example 3: Creating a Reusable Function Check

Instead of writing the typeof condition repeatedly, we can create a helper function that checks whether a value is a function.

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

function fun() {}

if (isFunction(fun)) {
    console.log("Function is defined");
} else {
    console.log("Function is not defined");
}

Function is defined

• typeof possibleFunction determines the type of the provided value.
• If the result is "function", isFunction() returns true.
• Otherwise, it returns false.
• This makes the function reusable for checking different values.