Creating our own advanced Array methods: forEach, map, filter, reduce, every and some, using native javascript code, so that we can understand them better.

Advanced Array methods: Native javascript code for forEach, map, filter, reduce, every, some

//forEach

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




//map

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




//filter

Array.prototype.myFilter = function(cb) {
    let newArr = [];
    for (let i = 0; i < this.length; i++) {
        if (cb(this[i], i, this)){
            newArr.push(this[i]);
         }
    }
    return newArr;
};




//some

Array.prototype.mySome = function(cb) {
    for (let i = 0; i < this.length; i++) {
        if (cb(this[i], i, this)){
            return true;
        }
    };
    return false;
};





//every

Array.prototype.myEvery = function(cb) {
    for (let i = 0; i < this.length; i++) {
        if (!cb(this[i], i, this)){
            return false;
        }
    };
    return true;
};



//reduce

Array.prototype.myReduce = function(cb, initialVal) {
    var accumulator = (initialVal === undefined) ? undefined : initialVal;
    for (var i = 0; i < this.length; i++) {
        if (accumulator !== undefined){
            accumulator = cb.call(undefined, accumulator, this[i], i, this);
        } else {
            accumulator = this[i];
         }
    }
    return accumulator;
};

The second line in myReduce is redundant. You could just put var accumulator = initialVal; or used initialVal in for loop instead of accumulator, it would do the same.
I would change it to the following implementation:

Array.prototype.myReduce = function(cb, reducer) {
    const {length} = this;
    let i = 0;
    if (!reducer) {
        reducer = this[0];
        i++;
    }
    for (; i < length; i++) {
        reducer = cb(reducer, this[i], i, this)
    }
    return reducer;
}

Other than that - great job of putting it together!

PS If you change file extension to .js you'll have awesome syntax highlighting ;)

thanks Dmitry!