实现数组的push、filter、map方法

1、实现push方法

let arr = [];
Array.prototype.push = function() {
    for( let i = 0 ; i < arguments.length ; i++){
        this[this.length] = arguments[i] ;
    }
    return this.length;
}

2、实现filter方法

Array.prototype.filter = function(fn) {
   if (typeof fn !== "function") {
       throw Error('请传入一个函数');
   }
   const res = [];
   for (let i = 0, len = this.length; i < len; i++) {
       fn(this[i]) && res.push(this[i]);
   }
   return res;
}

3、实现map方法

Array.prototype.map = function(fn) {
   if (typeof fn !== "function") {
        throw Error('参数是函数');
    }
    const res = [];
    for (let i = 0, len = this.length; i < len; i++) {
        res.push(fn(this[i]));
    }
    return res;
}