js 实现reduce、forEach、map

reduce

Array.prototype.myReduce = function(fn,prev) {
    for(let i = 0;i< this.length;i++){
        if(typeof prev === 'undefined') {
            prev = fn(this[i],this[i+1],i+1,this)
            ++i;
        }else{
            prev = fn(prev,this[i],i,this)
        }
    }
    return prev
}

forEach

Array.prototype.myForEach = function(fn){
    for(let i = 0;i<this.length;i++){
        fn(this[i],i)
    }
}

map

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