promise 的使用

回调函数

//回调函数：将后续处理的逻辑传入当前要做的事中，事情做好后调用后续处理逻辑


let a = '';

function get(callback) {
    setTimeout(()=>{
        a = 'hello word';
        callback(a);
    },2000)

}

function fun(val) {
    console.log(val);
}

get(fun)

promise

//Promise
/*
* resolve代表的是转向成功态
* reject 代表的是转向失败态 resolve和reject均是函数
* promise的实例就一个then方法，then方法中有两个参数
* */
let p = new Promise(((resolve, reject) => {
    setTimeout(()=>{
        let a = '鸡腿';
        resolve(a);  //失败调用reject(a)
    });
}));
p.then(data=>{
    console.log(data);
},err=>{
    console.log(err);
});

promise 抛出函数

//案列，在函数中抛出Promise实例

function buy() {
    return new Promise(((resolve, reject) => {
        setTimeout(() => {
            if (Math.random() > 0.5) {
                resolve('买');
            } else {
                reject('不买')
            }
        }, Math.random() * 1000);
    }));
}
buy().then(data=>{
    console.log(data)
},err=>{
        console.log(err);
    }
);

console.log('hello word')

nodejs 数组方法

console.log("hello word!");
let arr = [1, 2, 3, 4, 5,607];
arr.forEach((item,index)=>{
    if (index > 3) {
        return;     //注意forEach不支持ruturn
    }
    console.log(item, index);})//es5用来快速遍历数组

for (let arrKey in arr) {
    console.log(arrKey); //数组私有属性也会遍历出来
    //console.log(arr[arrKey])
}
let person ={age:12, name: "jingfu"}
for (let personKey in person) { //es5用来遍历对象属性的
    console.log(person[personKey]);
}
console.log("--------------")
for (let value of arr) {   //es6提供专门用来遍历数组'支持return
    // if (value == 3)
    //     { // noinspection JSAnnotator
    //         return;
    //     }
    console.log(value);

}

let newArr = arr.filter(item=>item>3)
console.log(arr,newArr)
let newArr2=arr.map(item=>item+2)
console.log(arr,newArr2)
//console.log(false<5)
let result=arr.find(item=>{
    return item.toString().indexOf('7')>-1
})
console.log("find arr结果为："+result);

let isEixed = arr.some(item=>{
    return item==4
});
//console.log(arr.includes(4))

let sum=arr.reduce((previousValue, currentVal, index, arr) => {
    console.log(previousValue,currentVal,index,arr)
    //return total+=currentVal;
    return previousValue+currentVal; //return结果作为下一次total
    },0);
console.log("arr累加后结果为："+sum)

let fruits = [{'count':3,price:5},{'count':2,price:5},{'count':3,price:9}];
let prices = fruits.reduce((total,currentVal)=>{
    console.log(total,currentVal)
    return total+=currentVal.count*currentVal.price;
},0)
console.log('总价为：'+prices);

//利用reduce数组去重
let arr3 = [1, 2, 3, 4, 5, 2, 1];
let newArr3 = arr3.reduce((pre,cur)=>{
    console.log(pre,cur)
    if(!pre.includes(cur)){
        return pre.concat(cur)
    }else{
        return pre
    }
},[]);
console.log("arr2去重后结果为："+newArr3)