This article brings you an introduction to the basic usage of Promise in ES6. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.
Promise is a solution for asynchronous programming
1.es5 callback
{
let ajax = function (callback) {
console.log('run1');
setTimeout(function () {
callback && callback.call();
}, 1000);
};
ajax(function () {
console.log('timeout1');
});
// 结果：立马输出 run1，1秒之后输出 timeout1
// 1.如果过程变得复杂，a()=>b()=>c()=>d()... ，用回调来处理会使代码变得复杂
// 2.代码复杂影响后期维护，很难一眼看出回调之间的顺序问题，代码难以阅读
}2.es6 Promise
{
let ajax = function () {
console.log('run2');
// 返回一个对象（Promise实例），这个实例具有then()方法，就是执行下一步的功能
return new Promise(function (resolve, reject) {
// resolve表示执行下一步操作 reject表示中断当前操作
setTimeout(function () {
resolve();
}, 1000);
});
};
ajax().then(function () {
// 这个方法对应resolve
console.log('timeout2');
}, function () {
// 这个方法对应reject
});
// 结果：立马输出 run2，1秒之后输出 timeout2
// 结果和es5一样，但是代码可读性、可维护性更高
}3.The series operation of then()
{
let ajax = function () {
console.log('run3');
return new Promise(function (resolve, reject) {
setTimeout(function () {
resolve();
}, 2000);
});
};
ajax().then(function () {
// 返回Promise实例，通过不断的下一步下一步，实现了串联操作的结果
return new Promise(function (resolve, reject) {
setTimeout(function () {
resolve();
}, 3000);
});
}).then(function () {
console.log('timeout3');
});
// 结果：立马输出 run3，5秒之后输出 timeout3
}4.catch method
{
let ajax = function (num) {
console.log('run4');
return new Promise(function (resolve, reject) {
if (num >= 5) {
resolve();
} else {
throw Error(`出错了，${num}小于5`);
}
});
};
// catch方法用来捕获异常
ajax(6).then(function () {
console.log(6);
}).catch(function (err) {
console.log(err);
});
// 结果：输出 run4 6
ajax(3).then(function () {
console.log(3);
}).catch(function (err) {
console.log(err);
});
// 结果：输出 run4 Error: 出错了，3小于5
}Example: Add all images to the page after loading
{
function loadImg(src) {
return new Promise((resolve, reject) => {
let img = document.createElement('img');
img.src = src;
// onload监听图片加载完成
img.onload = function () {
resolve(img);
};
// onerror监听图片加载失败
img.onerror = function (err) {
reject(err);
};
});
}
function showImgs(imgs) {
console.log(imgs); // [img, img, img] 三个img对象
imgs.forEach(function (img) {
document.body.appendChild(img);
});
}
// Promise.all 把多个Promise实例当做一个Promise实例
// Promise.all(array).then(fn),array对应多个Promise实例，当所有实例状态发生改变时，新的Promise实例才会跟着发生变化
// Promise.all也是返回Promise实例，所以也有.then()方法
// 当三张图片都加载完成，才会执行showImgs，如果其中有图片正在加载，不会执行showImgs，页面上看不到任何一张图片
Promise.all([
loadImg('https://img3.doubanio.com/view/photo/s_ratio_poster/public/p2530513100.webp'),
loadImg('https://img3.doubanio.com/view/photo/s_ratio_poster/public/p2529571873.webp'),
loadImg('https://img1.doubanio.com/view/photo/s_ratio_poster/public/p2395733377.webp')
]).then(showImgs);
}Example: Add one image to the page after loading, and ignore the rest , only the first picture to be loaded is displayed
{
function loadImg(src) {
return new Promise((resolve, reject) => {
let img = document.createElement('img');
img.src = src;
// onload监听图片加载完成
img.onload = function () {
resolve(img);
};
// onerror监听图片加载失败
img.onerror = function (err) {
reject(err);
};
});
}
function showImgs(img) {
let p = document.createElement('p');
p.appendChild(img);
document.body.appendChild(p);
}
// 当某一张图片率先加载完成，执行showImgs显示这张图片，剩余图片不再响应
Promise.race([
loadImg('https://img3.doubanio.com/view/photo/s_ratio_poster/public/p2530513100.webp'),
loadImg('https://img3.doubanio.com/view/photo/s_ratio_poster/public/p2529571873.webp'),
loadImg('https://img1.doubanio.com/view/photo/s_ratio_poster/public/p2395733377.webp')
]).then(showImgs);
}
#
posted on
Posted 1 day ago
javascript
es6
19 Reading It takes 15 minutes to read
Basic usage of PromisePromise is an asynchronous programming solution1.es5 callback
{
let ajax = function (callback) {
console.log('run1');
setTimeout(function () {
callback && callback.call();
}, 1000);
};
ajax(function () {
console.log('timeout1');
});
// 结果：立马输出 run1，1秒之后输出 timeout1
// 1.如果过程变得复杂，a()=>b()=>c()=>d()... ，用回调来处理会使代码变得复杂
// 2.代码复杂影响后期维护，很难一眼看出回调之间的顺序问题，代码难以阅读
}2.es6 Promise
{
let ajax = function () {
console.log('run2');
// 返回一个对象（Promise实例），这个实例具有then()方法，就是执行下一步的功能
return new Promise(function (resolve, reject) {
// resolve表示执行下一步操作 reject表示中断当前操作
setTimeout(function () {
resolve();
}, 1000);
});
};
ajax().then(function () {
// 这个方法对应resolve
console.log('timeout2');
}, function () {
// 这个方法对应reject
});
// 结果：立马输出 run2，1秒之后输出 timeout2
// 结果和es5一样，但是代码可读性、可维护性更高
}3.Then() series operation
{
let ajax = function () {
console.log('run3');
return new Promise(function (resolve, reject) {
setTimeout(function () {
resolve();
}, 2000);
});
};
ajax().then(function () {
// 返回Promise实例，通过不断的下一步下一步，实现了串联操作的结果
return new Promise(function (resolve, reject) {
setTimeout(function () {
resolve();
}, 3000);
});
}).then(function () {
console.log('timeout3');
});
// 结果：立马输出 run3，5秒之后输出 timeout3
}4.catch method
{
let ajax = function (num) {
console.log('run4');
return new Promise(function (resolve, reject) {
if (num >= 5) {
resolve();
} else {
throw Error(`出错了，${num}小于5`);
}
});
};
// catch方法用来捕获异常
ajax(6).then(function () {
console.log(6);
}).catch(function (err) {
console.log(err);
});
// 结果：输出 run4 6
ajax(3).then(function () {
console.log(3);
}).catch(function (err) {
console.log(err);
});
// 结果：输出 run4 Error: 出错了，3小于5
}Example: add all images to the page after loading
{
function loadImg(src) {
return new Promise((resolve, reject) => {
let img = document.createElement('img');
img.src = src;
// onload监听图片加载完成
img.onload = function () {
resolve(img);
};
// onerror监听图片加载失败
img.onerror = function (err) {
reject(err);
};
});
}
function showImgs(imgs) {
console.log(imgs); // [img, img, img] 三个img对象
imgs.forEach(function (img) {
document.body.appendChild(img);
});
}
// Promise.all 把多个Promise实例当做一个Promise实例
// Promise.all(array).then(fn),array对应多个Promise实例，当所有实例状态发生改变时，新的Promise实例才会跟着发生变化
// Promise.all也是返回Promise实例，所以也有.then()方法
// 当三张图片都加载完成，才会执行showImgs，如果其中有图片正在加载，不会执行showImgs，页面上看不到任何一张图片
Promise.all([
loadImg('https://img3.doubanio.com/view/photo/s_ratio_poster/public/p2530513100.webp'),
loadImg('https://img3.doubanio.com/view/photo/s_ratio_poster/public/p2529571873.webp'),
loadImg('https://img1.doubanio.com/view/photo/s_ratio_poster/public/p2395733377.webp')
]).then(showImgs);
}Example: One picture is added to the page after it is loaded, and the rest is ignored. Only the first picture to be loaded is displayed.
{
function loadImg(src) {
return new Promise((resolve, reject) => {
let img = document.createElement('img');
img.src = src;
// onload监听图片加载完成
img.onload = function () {
resolve(img);
};
// onerror监听图片加载失败
img.onerror = function (err) {
reject(err);
};
});
}
function showImgs(img) {
let p = document.createElement('p');
p.appendChild(img);
document.body.appendChild(p);
}
// 当某一张图片率先加载完成，执行showImgs显示这张图片，剩余图片不再响应
Promise.race([
loadImg('https://img3.doubanio.com/view/photo/s_ratio_poster/public/p2530513100.webp'),
loadImg('https://img3.doubanio.com/view/photo/s_ratio_poster/public/p2529571873.webp'),
loadImg('https://img1.doubanio.com/view/photo/s_ratio_poster/public/p2395733377.webp')
]).then(showImgs);
}#report
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