How to Check if a Function Exists in JavaScript With typeof

If you try to call a function that doesn’t exists, JavaScript will throw an error “Uncaught ReferenceError: function is not defined”. To prevent this error, you can typeof to check if a function exists before calling it.

What is typeof Operator

The typeof operator returns a string indicating the type of the unevaluated operand. Below is a list of possible values it will return:

• symbol
• string
• bigint
• number
• boolean
• undefined
• function

In another word, typeof will only return “function” if the JavaScript function exists.

How to Use typeof to Check if a Function Exists

typeof is very easy to use. Lete’s look at the following example, we have a JavaScript function called displayMessage, and we are checking if typeof operator returns “function” before calling it.

// check if function exists
function displayMessage(){
  console.log("typeof Example");
}

if(typeof printMessage === "function"){
  displayMessage();
}

How to Check if a Function Exists in Parent Window

typeof operator can be use to check a function in the parent windows, see below for an example:

// check if function exists in parent windows
if(typeof parent.printMessage === "function"){
  displayMessage();
}