How to Check If a Function Exists in JavaScript

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To check if a function exists in JavaScript, you can use either the “typeof” operator or the “try-catch block”.

Method 1: Using the typeof operator

The typeof operator checks whether the name of the declared function exists and whether it is a function and not some other type of object or primitive.

Syntax

if (typeof name_of_function === 'function') {
   name_of_function();
}

Example 1

function mainFunction() {
  console.log('Hello, world!');
}

if (typeof mainFunction === 'function') {
  mainFunction(); // Calls the function if it exists
} else {
  console.log('The function does not exist.');
}

Output

In this example, typeof mainFunction will return the string ‘function’ if mainFunction is a function.

If it’s not a function or not defined, the typeof mainFunction will return a different string (e.g., ‘undefined’ or ‘object’).

By checking if typeof mainFunction === ‘function’, you can ensure that you only call the function if it exists.

Example 2

You can use a similar approach if you work with object methods or functions in a particular namespace.

const mainObject = {
  mainMethod: function () {
    console.log('Hello, world!');
  },
};

if (typeof mainObject.mainMethod === 'function') {
  mainObject.mainMethod(); // Calls the method if it exists
} else {
  console.log('The method does not exist.');
}

Output

In this example, the function mainMethod is a property of the mainObject object.

By checking if typeof mainObject.mainMethod === ‘function’, you can ensure that you only call the method if it exists.

Method 2: Using the try-catch block

The try…catch block handles errors likely to occur within that block. For example, we will use this approach to handle the undefined error we expect JavaScript to throw when we call a function that we have not defined.

Syntax

try {
  mainFunction();
} 
catch(err) {
  console.log(err);
}

Example

function mainFunction() {
  console.log('Hello, world!');
}

try {
  mnFunction();
}
catch (err) {
  console.log(err);
}

Output

ReferenceError: mnFunction is not defined

You can see that the mnFunction() is not defined and got the error message output to the console.

Bonus method: Using an if-else statement

The if-else approach is beneficial if you use JavaScript in the browser. However, if you use this approach on Node.js, it won’t work because the window object does not exist in Node.js but only in Browser.

We will check the function as a method of the “window” object.

Syntax

if (window.mainFunction) {
   // ... 
}

Example

function mainFunction() {
  console.log('Hello, world!');
}

if (window.mainFunction) {
  console.log('The mainFunction() function is defined');
}
else {
  console.log('The mainFunction() function is not defined');
}

Output

The mainFunction() function is defined

That’s it.