Check if function exists in Java script

You may see undefined errors in the console when you try to call a function that is not defined in JavaScript code. JavaScript throws this error and stops running the code.

In this article, I will teach you how to check whether a function exists. This way you avoid any possible mistakes. This is a useful technique for seeing if a specific library or API is available in the client you're running your software on.

JavaScript has a few different ways to see if a function exists. I'll show you a few.

Use if conditional statement

One way to check if a function is defined is to test it using an if statement. The trick is to test the function as a method of the window object.

So, if you want to test aFunctionName, just use:

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

If the function is defined, the code in the brackets will be executed. In contrast, if you just test a function without using a window object, such as if(aFunctionName), JavaScript will throw a ReferenceError if the function does not exist.

Let us consider the following example that checks the existence of two functions: one exists and the other does not.

// Testing a function that exists
function exists() {
    // ...
}

if (window.exists) {
    console.log('the exists() function exists');
}
else{
    console.log('the exists() function does not exist');
}

if(window.doesntExist) {
    console.log('the doesntExist() function exists');
}
else{
    console.log('the doesntExist() function does not exist');
}

The above code snippet will output:

the exists() function exists
the doesntExist() does not exist

This works great for our example, but one problem with this approach is that we don't check whether the named object is actually a function. In fact, any variable with the same name will fool our test into thinking the function is defined.

typeof Operator

Alternatively, we can use the typeof operator. This operator will check whether the name of the declared function exists and whether it is a function and not some other type of object or primitive.

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

In the above example, we test whether nameOfFunction exists and run it if it exists.

Use Try...catch block

try…catch block handles errors that may occur within the block. We will use this method to handle undefined errors that JavaScript throws when we call an undefined function.

try...catch How the statement works

We run the function inside a try block. If it does not exist, an exception is thrown and handled by the catch block.

This is an example:

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

If testFunction is not defined, the following message will be output to the console.

ReferenceError: testFunction is not defined

This is also what we see without the try...catch block, but in this case our code will continue to run below the catch block.

in conclusion

This article describes the three main ways to check whether a function exists in JavaScript before calling it. These are the use of the if conditional statement, the use of the typeof operator, and finally the try...catch statement. I also use examples to explain how JavaScript implements these methods to check if a function exists. I hope this concept is clearer to you now!

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