SOURCE

Array.prototype.myMap = function (fn) {
    return this.reduce((pre, cur, i, arr) => {
        pre.push(fn.call(this, cur, i, arr))
        return pre;
    }, [])
}

const arr =[1,2,3];
const arr2 = arr.myMap(function(item,i,arr){
    return item*2
});
console.log(arr2);
//输出[2,4,6]