手写forEach、map、reduce、filter

[TieMuZhen]

一、forEach

代码实现

Array.prototype.myForeach = function(callback, thisArg = null){
    if(typeof callback != "function"){
        throw new TypeError(callback + " is not a function");
    }
    for(let i = 0; i < this.length; i++){
        callback.call(thisArg, this[i], i, this);
    }
}

测试

["a", "b", "c"].myForeach(function(item, index, arr){
    console.log(item, index, arr);
})

二、map

代码实现

Array.prototype.myMap = function(callback, thisArg = null){
    if(typeof callback != "function"){
        throw new TypeError(callback + " is not a function");
    }
    let res = new Array(this.length);
    for(let i = 0; i < this.length; i++){
        let data = callback.call(thisArg, this[i], i, this);
        res.push(data);
    }
    return res;
}

测试

let res = [1, 2, 3].myMap(function(item, index, arr){
    return item + 2;
})
// [3, 4, 5]

三、reduce

代码实现

Array.prototype.myReduce= function(fn, init){
    for(let i = 0; i < this.length; i++){
        if(init){
            init = fn.call(null, init, this[i], i, this);
        }else{
            init = fn.call(null, this[i], this[i + 1], i + 1, this);
            i++;
        }
    }
    return init;
}

测试

let sum = [1, 2, 3].myReduce(function(prev, next, currentIndex, array){
    return prev + next;
});
// 6

四、filter

代码实现

Array.prototype.myFilter = function(callback, thisArg = null){
    if(typeof callback != "function"){
        throw new TypeError(callback + " is not a function");
    }
    let arr = []
    for(let i = 0; i < this.length; i++){
        callback.call(thisArg, this[i]) && arr.push(this[i]);
    }
    return arr;
}

测试

let res = [1, 2, 3].myFilter(function(item){
    return item > 2;
});
console.log(res); // [3]