数组常见方法

push

Array.prototype.push=function(...items){
    // 确保 this 是一个对象
    let O=Object(this);
    // >>> 是无符号右移操作符
    // >>> 0 的作用是将 length 转换为一个合法的非负整数
    let len=this.length>>>0;
    let argCount=items.length>>>0;
    // 2**53-1是JS能表示的最大正整数
    if(len+argCount>2**53-1){
        throw new TypeError('The number of array is over the max value');
    }
    for(let i=0;i<argCount;i++){
        O[len+i]=items[i];
    }
    let newLength=len+argCount;
    O.length=newLength;
    return newLength;
}

pop

Array.prototype.pop=function(){
    let O=Object(this);
    let len=this.length>>>0;
    if(len===0){
        O.length=0;
        return undefined;
    }
    len--;
    let value=O[len];
    delete O[len];
    O.length=len;
    return value;
}

map

Array.prototype.map=function(callbackFn,thisArg){
    if(this===null||this===undefined){
        throw new TypeError("Cannot read property 'map' of null");
    }
    if(Object.prototype.toString.call(callbackFn)!=='[object Function]'){
        throw new TypeError(callbackFn+'is not a function');
    }
    let O=Object(this);
    let T=thisArg;

    let len=O.length>>>0;
    let A=new Array(len);
    for(let k=0;k<len;k++){
        if(k in O){
            let kValue=O[k];
            // 依次传入this，当前项，当前索引，整个数组
            let mappedValue=callbackFn.call(T,kValue,k,O);
            A[k]=mappedValue;
        }
    }
    return A;
}

reduce

Array.prototype.reduce=function(callbackFn,initialValue){
    if(this===null||this===undefined){
        throw new TypeError("Cannot read property 'reduce' of null");
    }
    if(Object.prototype.toString.call(callbackFn)!=='[object Function]'){
        throw new TypeError(callbackFn+'is not a function');
    }
    let O=Object(this);
    let len=O.length>>>0;
    let k=0;
    let accumulator=initialValue;
    if(accumulator===undefined){
        for(;k<len;k++){
            if(k in O){
                accumulator=O[k];
                k++;
                break;
            }
        }
        throw new Error('Each element of the array is empty');
    }
    for(;k<len;k++){
        if(k in O){
            // 注意reduce的核心累加器
            accumulator=callbackFn(accumulator,O[k],k,O);
        }
    }
    return accumulator;
}