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Home  >  Article  >  Web Front-end  >  Introduction to the basic usage of Promise in ES6

Introduction to the basic usage of Promise in ES6

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2018-10-29 15:20:424084browse

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
}

Promise.all()

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);
}

Promise.race()

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);
}

         

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#                                                                                                                                                                                         posted on                                                                                                                                                                                                                                                                                                                                                                                                                                                                Posted 1 day ago Introduction to the basic usage of Promise in ES6

【ES6 Getting Started 12】：Promise

Introduction to the basic usage of Promise in ES6javascript

es6

front end

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
}
Promise.all()

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);
}
Promise.race()

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);
}

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