I see a lot of pros javascript developer testing if an object is a function using
Object.prototype.toString.call(fn) === "[object Function]"
Usually I am using
fn instanceof Function === true
can someone enlighten me. Thank you
instanceof just looks for the constructor property in the prototype chain. See the standard:
@@hasInstance: A method that determines if a constructor object recognizes an object as one of the constructor's instances. Called by the semantics of the instanceof operator.
Which means you can "trick" it:
let Constructor = function(){};
Constructor.prototype = Function.prototype;
let Not_A_Real_Function = function(){};
Not_A_Real_Function.prototype = new Constructor;
let not_a_real_function = new Not_A_Real_Function;
// instanceof
console.log(not_a_real_function instanceof Function);
// prints true
// Object.prototype.toString
console.log(Object.prototype.toString.call(not_a_real_function) === "[object Function]");
// prints false
// typeof
console.log(typeof not_a_real_function === "function");
// prints falseA downside of Object.prototype.toString however is that you can potentially override it all together, to make it return whatever you want.
let Object = {
prototype: {
toString(fn){
return "foo";
}
}
};
let fc = function(){};
console.log(Object.prototype.toString.call(fc) === "foo");For the sake of completeness:
As @Bergi pointed out,
typeof fn == "function"
is probably the safest way. (See the first snippet)