3 Ways to Check if a Function is Defined in JavaScript

How to Check if a Function is Defined in JavaScript?

When developing JavaScript applications, there may be cases where you need to check if a specific function is defined before calling it. This can help prevent runtime errors that occur when you try to invoke a function that hasn’t been defined yet.

In this blog post, we’ll explore the different ways to check if a function is defined in JavaScript and how to handle undefined functions gracefully.

Table of Contents

1. Checking if a function is defined using typeof
2. Checking with window or globalThis object
3. Handling undefined functions with try...catch
4. Example Scenarios

1. Checking if a Function is Defined Using typeof

One of the most straightforward and reliable ways to check if a function is defined is by using the typeof operator. In JavaScript, the typeof operator returns a string indicating the type of the operand. If the function is defined, typeof will return "function". If it’s not defined, it will return "undefined".

Here’s a simple example:

if (typeof myFunction === "function") {
    // Function is defined, we can safely call it
    myFunction();
} else {
    console.log("myFunction is not defined.");
}

Explanation:

• typeof myFunction checks the type of myFunction.
• If the result is "function", it indicates that myFunction is defined.
• Otherwise, "undefined" is returned, meaning that the function doesn't exist.

When to Use typeof

This approach is particularly useful when you are dealing with optional or dynamically loaded JavaScript code. It ensures that you won't encounter a runtime error by trying to invoke an undefined function.

2. Checking with window or globalThis Object

In a browser environment, functions defined in the global scope are properties of the window object. In a Node.js or more modern JavaScript environments, you can use globalThis to access the global object. You can check if a function exists as a property of these objects.

Example using the window object (for browsers):

if (typeof window.myFunction === "function") {
    // myFunction is defined
    myFunction();
} else {
    console.log("myFunction is not defined.");
}

Example using globalThis (for universal JavaScript):

if (typeof globalThis.myFunction === "function") {
    // myFunction is defined
    globalThis.myFunction();
} else {
    console.log("myFunction is not defined.");
}

Explanation:

• window.myFunction or globalThis.myFunction will reference the function if it’s globally defined.
• The same typeof check is applied to ensure it’s a function before calling it.

When to Use window or globalThis

This method is useful when you want to be explicit about the scope and make sure you are checking the global environment for the function.

3. Handling Undefined Functions with try...catch

Another way to handle potentially undefined functions is to use a try...catch block. This method allows you to safely attempt to call a function and catch any errors if the function is not defined.

try {
    myFunction(); // Try to call the function
} catch (e) {
    console.log("myFunction is not defined or cannot be executed.", e.message);
}

Explanation:

• try attempts to invoke the function.
• If the function is not defined, the code inside catch is executed, preventing the script from crashing.
• The error message can be logged or handled accordingly.

When to Use try...catch

This method is helpful when you want to handle not only undefined functions but also any other potential runtime errors that might occur when calling a function.

4. Example Scenarios

Example 1: Checking for a Plugin Function

Imagine you’re building a website that optionally uses a plugin. You may want to check if a plugin-specific function is available before trying to use it.

if (typeof jQuery.fn.myPlugin === "function") {
    // The plugin is available, proceed to use it
    $("div").myPlugin();
} else {
    console.log("myPlugin is not available.");
}

Example 2: Dynamically Loaded Functions

In a single-page application (SPA), you might load different scripts dynamically. Before calling a function from a script that may or may not have loaded, you can perform a check like this:

function callDynamicFunction() {
    if (typeof dynamicFunction === "function") {
        dynamicFunction();
    } else {
        console.log("dynamicFunction is not yet loaded.");
    }
}

Example 3: Fallback Logic

Sometimes, when a function is not defined, you might want to provide fallback logic or call an alternative function.

if (typeof customLogFunction === "function") {
    customLogFunction("This is a custom log message.");
} else {
    console.log("This is a fallback log message.");
}

Conclusion

In JavaScript, there are several ways to check if a function is defined:

• The typeof operator is the simplest and most widely used approach.
• You can also check the window or globalThis object to look for global functions.
• For more robust error handling, using try...catch ensures your application doesn’t break when calling undefined functions.

By incorporating these techniques, you can build more reliable and error-tolerant JavaScript applications. Always ensure that your checks are in place, especially in scenarios involving dynamic code or optional libraries.

Also read: Why typeof Returns 'Object' for JavaScript Arrays

Also read: How to Check if a Variable or Constant is Defined in JavaScript