Promise详解

本文主要通过对Promise的八大特性讲解，来详细探讨Promise。

1.立刻执行性

const p = new Promise((resolve, reject) => {
    console.log("create a promise");
    resolve("success");
});

console.log("after new Promise");
p.then((value) => {
    console.log(value);
});

执行结果:

分析：只要一创建Promise，就会立即执行Promise的内部函数

2.三大状态（Promise有且只有三种状态pending，resolved，rejected）

var p1 = new Promise((resolve, reject) => {
    resolve(1);
});
var p2 = new Promise((resolve, reject) => {
    setTimeout(function () {
        resolve(2);
    }, 500);
});
var p3 = new Promise((resolve, reject) => {
    setTimeout(function () {
        reject(3);
    }, 500);
});
console.log('0', p1);
console.log("1", p2);
console.log("2", p3);
setTimeout(() => {
    console.log("3", p2);
}, 1000);
setTimeout(function () {
    console.log("4", p3);
}, 1000);
p1.then((value) => {
    console.log("5", value);
});
p2.then((value) => {
    console.log("6", value);
});
p3.catch((err) => {
    console.log("7", err);
});

结果：

3.状态不可逆（Promise状态只能由pending到resolved或者由pending到rejected）

const p1 = new Promise(function (resolve, reject) {
    resolve("success1");
    resolve("success2");
});

const p2 = new Promise(function (resolve, reject) {
    resolve("success");
    reject("reject");
});

p1.then(function (value) {
    console.log(value);
});

p2.then(function (value) {
    console.log(value);
});

结果：

4.then回调异步性

const p = new Promise(function (resolve, reject) {
    resolve("success");
});

p.then(function (value) {
    console.log(value);
});

console.log("which one is called first ?");

输出结果：

5.支持链式调用

const p = new Promise((resolve, reject) => {
    resolve(1);
});
p.then((value) => {
    console.log(value);
    return value * 2;
})
.then((value) => {
    console.log(value);
})
.then((value) => {
    console.log(value);
    return Promise.resolve('resolve');
})
.then((value) => {
    console.log(value);
    return Promise.reject('reject');
})
.then((value) => {
    console.log('resolve: ' + value);
}, (err) => {
    console.log('reject: ' + err);
})

输出结果：

6.异常处理：

const p1 = new Promise( function(resolve,reject){
    foo.bar();
    resolve( 1 );      
});
p1.then((value)=>{
        console.log('p1 then value: ' + value);
    },
    (err)=>{
        console.log('p1 then err: ' + err);
    }
)
.then((value)=>{
        console.log('p1 then then value: '+value);
    },
    (err)=>{
        console.log('p1 then then err: ' + err);
    }
);
const p2 = new Promise((resolve,reject)=>{
    resolve( 2 );    
});
p2.then(
    (value)=>{
        console.log('p2 then value: ' + value);
        foo.bar();
    }, 
    (err)=>{
        console.log('p2 then err: ' + err);
    }
)
.then(
    (value)=>{
        console.log('p2 then then value: ' + value);
    },
    (err)=>{
        console.log('p2 then then err: ' + err);
        return 1;
    }
).then(
    (value)=>{
        console.log('p2 then then then value: ' + value);
    },
    (err)=>{
        console.log('p2 then then then err: ' + err);
    }
);

输出：

7.Promise.resolve

let p1 = Promise.resolve(1);
let p2 = Promise.resolve(p1);
let p3 = new Promise(function (resolve, reject) {
    resolve(1);
});
let p4 = new Promise(function (resolve, reject) {
    resolve(p1);
});
console.log(p1 === p2);
console.log(p1 === p3);
console.log(p1 === p4);
console.log(p3 === p4);
p4.then(function (value) {
    console.log('p4=' + value);
});
p2.then(function (value) {
    console.log('p2=' + value);
})
p1.then(function (value) {
    console.log('p1=' + value);
})

输出结果：

8.resolve和reject

const p1 = new Promise((resolve, reject)=>{
    resolve(Promise.resolve('resolve'));
});
const p2 = new Promise((resolve, reject)=>{
    resolve(Promise.reject('reject'));
});

const p3 = new Promise((resolve, reject)=>{
    reject(Promise.resolve('resolve'));
});

p1.then((value)=>{
        console.log('fulfilled: ' + value);
    }, 
    (err)=>{
        console.log('rejected: ' + err);
    });
p2.then((value)=>{
        console.log('fulfilled: ' + value);
    }, 
    (err)=>{
        console.log('rejected: ' + err);
    });
p3.then((value)=>{
        console.log('fulfilled: ' + value);
    },
    (err)=>{
        console.log('rejected: ' + err);
    });

输出结果：