JavaScript Check if Function Exists

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JavaScript gives us a method to check if a function exists or not. We can check it using the typeof operator and an if condition.

Using typeof Operator

To check if a function exists in JavaScript or not, we can use the typeof operator. This operator will check whether the name of the declared function exists and also whether it is a function or not. Here is a syntax:

Syntax:

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

To understand it clearly, let’s write a code to check, if the hello function exists or not.

Example

<script>
function hello() {
  document.write("This text will be displayed if a function exists")
}
if (typeof hello === "function") {

  hello();
} else {
  document.write("hello is not defined");
}
</script>

Since the hello function exists in the code, It will execute the function.

Example Explained

In an above example, we are checking if the hello function is defined or not using the typeof operator. Since the hello function is defined, it returns the “function” and the code inside the if block will be executed. If hello was not defined, it will return undefined and the code inside the else block will be executed.

Let’s take a look at another example:

Example

<script>
function hello() {
  document.write("This text will be displayed if a function exists")
}
if (typeof bye === "function") {

  bye();
} else {
  document.write("bye is not defined");
}
</script>

In this example, we are checking if the bye function is defined or not. Since the bye function is not defined, it will not return the “function” and the code inside the else block will be executed.

Run this code on our Online Code Editor to see how it works.

Using if Condition

Another way to check if a JavaScript function exists or not is by using an if conditional statement.

Here we have to test the function as a method of the window object. Here is a syntax:

Syntax

if (window.myFunction) {
    // myFunction is defined

myFunction();
}
else {
  // myFunction is not defined
console.log("myFunction is not defined");
}

To understand it clearly, let’s write a code to check, if the world function exists or not.

Example

<script>
function world() {
    document.write("World function");
}
if (window.world) {
world();
}
else {
  document.write("world function does not exist")
}
</script>

In an above example, we are checking if the world function is defined or not. Since the world function is defined, it returns the “function” and the code inside the if block will be executed. If the world was not defined, it will return undefined and the code inside the else block will be executed.

Let’s take a look at the another example and here we check the function that doesn’t exist.

Example

<script>
function world() {
    document.write("World function");
}
if (window.earth) {
world();
}
else {
  document.write("earth function does not exist")
}
</script>

In an above example, we are checking if the earth function is defined or not. Since the earth function is not defined, the code inside the else block will be executed.

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