How to Check if a Function Exists in JavaScript

Because JavaScript is a loosely-typed language, it can sometimes be unclear what type a variable is.

In the case of functions, if you call a function that is non-existent, you will get an error.

To avoid this, you can check if the function exists before calling it.

In this post, we'll learn how you can check if a function exists before calling it in JavaScript.

Checking if a function exists in JavaScript

First, let's define a simple function:

JAVASCRIPT
const greeting =(name => {
    console.log(`Hello ${name}`);
}

We know this is a function because we can call it:

JAVASCRIPT
const greeting = name => {
    console.log(`Hello ${name}`);
}

greeting("John");

BASH
Hello John

However, what if we want to check if the function exists before calling it?

To do this, you can try using the typeof operator:

JAVASCRIPT
const greeting = name => {
    console.log(`Hello ${name}`);
}

if (typeof greeting === "function") {
    console.log("This is a valid function")
}

BASH
This is a valid function

Another alternative you could do is simply try calling the function anyways but wrapping it in a try-catch block to catch any potential exceptions that might be thrown:

JAVASCRIPT
const greeting = name => {
    console.log(`Hello ${name}`);
}

try {
    greeting("John");
} catch (error) {
    console.log(error)
}

BASH
Hello John

If the function was not valid, an error would get printed to the console but the script would continue executing.

Conclusion

In this post, we learned how to check if a function exists in JavaScript.

Simply use the typeof operator or try calling the function within a try-catch block for safety.

Thanks for reading!

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