How to determine that a JavaScript function is native (without testing '[native code]')

Question

I would like to know if there is a way to differentiate a JavaScript script function (function(){}) from a JavaScript native function (like Math.cos

Answer 1

You could try to use a Function constructor with the toString value of the function. If it does not throw an error, then you get a custom function, otherwise you have a native function.

function isNativeFn(fn) {
    try {
        void new Function(fn.toString());    
    } catch (e) {
        return true;
    }
    return false;
}

function customFn() { var foo; }

console.log(isNativeFn(Math.cos));          // true
console.log(isNativeFn(customFn));          // false
console.log(isNativeFn(customFn.bind({}))); // true, because bind