Why you need to use typeof to check if a function exists with vanilla JS

A couple of days ago I shared a technique for checking if a function exists before trying to run it.

if (typeof getPurchases === 'function') {
	getPurchases(123);
}

One astute reader asked why we can’t just do this:

if (getPurchases) {
	getPurchases(123);
}

Great question!

That works in some cases, but… if the variable was assigned to something other than a function—a string, number, or object, for example—you’d the if statement would validate as true and then you’d try to run something that wasn’t a function and get an error.

var myFunction = 'Some string';
// or...
var myFunction = 123;

// This validates as true
if (myFunction) {
    // Since myFunction isn't a function, this would throw an error
    myFunction();
}