Array.prototype.forEach native implementation issue [duplicate]

Question

I'm trying to implement native forEach method. Here's my code:

Array.prototype.myEach = function(cb) {
     for(let i=0; i<this.length; i++) {
         cb(this[i], i)
     }
}

If I declare let a = [] (something) and then run [].myEach then it works.

let a = [1,2,3,4,5]; // or even []

[1,2,3,4,5].myEach(function(val, i){
    console.log(val); //works
});

But if I don't declare the array on the top, it's not even recognizing the prototype.

[1,2,3,4,5].myEach(function(val, i){ //fails
    console.log(val);
});

Problem:

If I remove let a = [1,2,3,4,5], doing [1,2,3,4].forEach fails.

I'm not able to understand why.

Answer 1

Include a semi-colon after the myEach function:

Array.prototype.myEach = function(cb) {
     for(let i=0; i<this.length; i++) {
         cb(this[i], i)
     }
};

[1,2,3,4,5].myEach(function(val, i){ 
    console.log(val);
});

Without the semi-colon, this parses like this:

Array.prototype.myEach = function(cb) {
     for(let i=0; i<this.length; i++) {
         cb(this[i], i)
     }
}[1,2,3,4,5].myEach( ....etc

and [1,2,3,4,5] attempts to obtain property 5 from the function (object) - as if you had written function(){}[5]. There is no property 5 and so this is undefined and attempting to call myEach on undefined gives an error.

The original worked because the intermediate let statement achieved the separation (thanks to the semi-colon but irrelevant of the actual let a = statement).