算是檢驗自己對JavaScript理解一個很好的方法:

範例 :

const cat1 = {
    name:"咪咪",
    age:2,
    color:"白色"
}

const cat2 = {
    name:"妹妹",
    age:1,
    color:"虎斑"
}

const cat3 = {
    name:"Toby",
    age:2,
    color:"虎斑"
}

const cats = [cat1,cat2,cat3]

1. forEach

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


cats.ownForEach((cat) => {
    console.log(cat)
})

//
{name: "咪咪", age: 2, color: "白色"}
{name: "妹妹", age: 1, color: "虎斑"}
{name: "Toby", age: 2, color: "虎斑"}

2. map

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

console.log(cats.ownMap((item) => `一隻${item.color}的貓`))
// ["一隻白色的貓", "一隻虎斑的貓", "一隻虎斑的貓"]

3. filter

Array.prototype.ownFilter = function(callback) {
    const result = [];
    for(let i = 0; i < this.length; i++) {
        if(callback(this[i],i,this)) {
            result.push(this[i]);
        }
    }
    return result;
}

console.log(cats.ownFilter((item) => item.color == '虎斑'))
// 
0: {name: "妹妹", age: 1, color: "虎斑"}
1: {name: "Toby", age: 2, color: "虎斑"}

這邊有另外一個寫法 :

Array.prototype.ownFilter = function(callback) {
    const result = [];
    for(let i = 0; i < this.length; i++) {
        callback(this[i],i,this) && result.push(this[i]) // 前者true就會執行後者
    }
    return result;
}

4. every

Array.prototype.ownEvery = function(callback) {
    let isAllTrue = true;
    for(let i = 0; i < this.length; i++) {
        isAllTrue = callback(this[i],i,this);
        if(!isAllTrue) break;
    }
    return isAllTrue;
}

console.log(cats.ownEvery((item) => item.age === 1)); // false

5. some

Array.prototype.ownSome = function(callback) {
    let isTrue = false
    for(let i = 0; i < this.length; i++) {
        isTrue = callback(this[i],i,this);
        if(isTrue) break
    }
    return isTrue;
}
console.log(cats.ownSome((item) => item.color === '黑色')); //false

6. reduce

Array.prototype.ownReduce = function(callback, initValue) {
    let startIndex = 0;
    let pre;
    if(initValue || initValue===0) {
        pre = initValue
    } else {
        pre = this[0];
        startIndex = 1;
    }
    
    for(let i = startIndex; i < this.length; i++) {
        pre = callback(pre, this[i], this)
    }
    return pre
}

const ageTotal = cats.ownReduce((pre,cur) => pre + cur.age,0); 
console.log(ageTotal); // 5