The difference between customize function and native function in prototype

Question

'For In' can traverse Array (value/properties/function).

let arr = [1,2,3,4];
arr.__proto__.con = function(){console.log('from array');}
for(let item in arr){
 console.log(item);
}

The result will be:

1,2,3,4,con

why does native function like 'toString'/'split' not be printed? what is the difference?

In my mind, they(con and toString) all belong to the prototype of Array.

Answer 1

The for...in statement...

iterates over all non-Symbol, enumerable properties of an object. (source: MDN)

However, if you look at the ECMA specification, in the item "Properties of the Array Prototype Object", you'll see that:

The Array prototype object is the intrinsic object %ArrayPrototype%. The Array prototype object is an Array exotic objects and has the internal methods specified for such objects. It has a length property whose initial value is 0 and whose attributes are { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.

That applies to concat, filter, map, forEach, sort, toString etc...

You can use Object.getOwnPropertyDescriptor to check those attributes. For instance:

console.log(Object.getOwnPropertyDescriptor(Array.prototype, "concat"));

Finally, for setting those methods with {enumerable: false}, have a look at the other answer.

Answer 2

It doesn't matter that the method is assigned to the prototype. Properties of any type of object are enumerable by default, whether they are assigned to the object's prototype or directly to the object:

class MyClass {
    constructor() {
        this.x = 10;
    }
}
let test = new MyClass();     
test.y = 20;
for(let item in test) {console.log(item) } // logs x, y

Native methods like toString are set to be non-enumerable, for the obvious reason that you don't often want to loop through them. User methods or properties can also be set to be non-enumerable, by using Object.defineProperty:

Object.defineProperty(test, "z", {value: 42, enumerable: false})
console.log(test.z);           // 42
for(let item in test) {console.log(item)}   // still only x, y