How to Check if a Function Exists in JavaScript

In JavaScript, it’s important to check if a function exists before calling it. This helps avoid errors and allows you to handle different scenarios appropriately. In this article, we will explore various methods to check whether a function exists in JavaScript, along with detailed explanations and examples.

Method 1: Using the typeof Operator

The typeof operator in JavaScript allows us to determine the type of a variable or expression. By using this operator on a function, we can check if it exists.

if (typeof functionName === 'function') {
  // function exists
} else {
  // function does not exist
}

Here, functionName is the name of the function you want to check. If the type of functionName is 'function', then the function exists; otherwise, it does not.

Method 2: Using the typeof Operator and Call

Another way to check if a function exists is by using the typeof operator in conjunction with the call method.

if (typeof functionName === 'function' && functionName.call) {
  // function exists
} else {
  // function does not exist
}

In this method, we first check if the type of functionName is 'function'. Then, we verify if the call method exists on that function. If both conditions are true, the function exists.

Method 3: Using the typeof Operator and Apply

Similar to the previous method, we can use the typeof operator along with the apply method to check if a function exists.

if (typeof functionName === 'function' && functionName.apply) {
  // function exists
} else {
  // function does not exist
}

Here, we perform the same checks as before but with the apply method instead of call.

Method 4: Using the typeof Operator and typeof window[functionName]

If the function you want to check is defined globally, you can use the typeof operator along with typeof window[functionName] to verify its existence.

if (typeof window[functionName] === 'function') {
  // function exists
} else {
  // function does not exist
}

In this approach, we access the function through the global window object using square brackets notation. By checking if its type is 'function', we can determine if it exists.

Method 5: Using the hasOwnProperty Method

If the function is defined within an object, you can use the hasOwnProperty method to check its existence.

if (objectName.hasOwnProperty('functionName')) {
  // function exists within the object
} else {
  // function does not exist within the object
}

Replace objectName with the name of the object containing the function and functionName with the name of the function itself. If objectName has a property named functionName, then the function exists within that object.

Conclusion

In this article, we explored different methods to check if a function exists in JavaScript. These methods will help you avoid errors and handle scenarios where functions may or may not be present. Remember to choose the appropriate method based on your specific use case.

By implementing these checks, you can ensure smoother execution of your JavaScript code and create more robust applications.