How to Check If a Function Exists in JavaScript

You may need to check if a function already exists before calling it.

To check if a function exists in JavaScript, you can use the typeof operator. The typeof operator tells you what type of the operand.

For example, if you use typeof with a function’s name and it says “function,” that’s how you know the function exists.

Here’s how to make it work:

if (typeof myFunction === 'function') { 
    // true
    console.log("The function exists.");
} else {
    // false
    console.log("The funciton does not exist.");
}

Remember, in JavaScript, a variable can be undefined which means that it has been declared but has not yet been assigned a value.

On the other hand, a variable can be undeclared which means that it has not been declared at all. If you try to access an undeclared variable, JavaScript will throw a ReferenceError.

The typeof operator treats undeclared variables as if they were declared with undefined value, so it won’t throw a ReferenceError even if the variable you are checking doesn’t exist at all.

Also, this will only check if a variable with the name myFunction exists in the current scope. If there is a function with that name in a different scope, this code won’t find it.

So make sure to check the correct scope.