promise体验

promise的执行流程

promise串行执行异步

job1.then(job2).then(job3).catch(handleError);

// 0.5秒后返回input*input的计算结果:
function multiply(input) {
    return new Promise(function (resolve, reject) {
        console.log('calculating ' + input + ' x ' + input + '...');
        setTimeout(resolve, 500, input * input);
    });
}
// 0.5秒后返回input+input的计算结果:
function add(input) {
    return new Promise(function (resolve, reject) {
        console.log('calculating ' + input + ' + ' + input + '...');
        setTimeout(resolve, 500, input + input);
    });
}

var p = new Promise(function (resolve, reject) {
    console.log('start new Promise...');
    resolve(123);
});

p.then(multiply)
 .then(add)
 .then(multiply)
 .then(add)
 .then(function (result) {
    console.log('Got value: ' + result);
});

promise并行执行异步任务(Promise.all())

• 假设此时有任务A和任务B两个异步任务,两个任务全部执行完成之后执行任务C,此时可以使用Promise.all()实现

var p1 = new Promise(function (resolve, reject) {
    setTimeout(resolve, 500, 'P1');
});
var p2 = new Promise(function (resolve, reject) {
    setTimeout(resolve, 600, 'P2');
});
var p3=function(res){
  console.log(res)
}
Promise.all([p1, p2]).then(p3);

promise并行执行多个任务,但是只需要得到先返回的结果即可(Promise.race())

• 假设此时有任务A和任务B两个异步任务,两个任务任意一个完成之后执行任务C,此时可以使用Promise.race()实现

var p1 = new Promise(function (resolve, reject) {
    setTimeout(resolve, 500, 'P1');
});
var p2 = new Promise(function (resolve, reject) {
    setTimeout(resolve, 600, 'P2');
});
var p3=function(res){
  console.log(res)
}
Promise.race([p1, p2]).then(p3);