21 # Implement the race method of promise

Race, whoever runs faster will use whoever (for example, multiple interface requests, use the fastest one)

const fs = require("fs").promises;

// 检查是否是 promise，不使用 instanceof 的原因是因为可能是别人实现的类
const isPromise = (value) => typeof value.then === "function";

Promise.race = function (promises) {
    
    
    return new Promise((resolve, reject) => {
    
    
        for (let i = 0; i < promises.length; i++) {
    
    
            let result = promises[i];
            if (isPromise(result)) {
    
    
                result.then(resolve, reject);
            } else {
    
    
                resolve(result);
            }
        }
    });
};

test:

Promise.race([fs.readFile("./file/name.txt", "utf-8"), fs.readFile("./file/age.txt", "utf-8")])
    .then((data) => {
    
    
        console.log("data--name--age-->", data);
    })
    .catch((err) => {
    
    
        console.log("err---->", err);
    });

Let's look at an application of race: interrupt promise A promise is on the way to success and succeeds after 3s. If it exceeds 2s, it is considered a failure

let promise = new Promise((resolve, reject) => {
    
    
    setTimeout(() => {
    
    
        resolve("ok 成功了");
    }, 3000);
});
// 通过 race 方法实现一个中断请求的包装方法
const wrap = (promise) => {
    
    
    let abort;
    // 构建一个新的 promise，跟传进来的竞赛
    let newP = new Promise((resolve, reject) => {
    
    
        abort = reject;
    });
    // 有一个失败就失败了
    let p = Promise.race([promise, newP]);
    p.abort = abort;
    return p;
};
let p = wrap(promise);
p.then(
    (data) => {
    
    
        console.log("data---->", data);
    },
    (err) => {
    
    
        console.log("err---->", err);
    }
);
setTimeout(() => {
    
    
    // 如果超过 2s 中断请求，中断就会触发，wrap 里构建的新的 promise 的 reject 方法
    p.abort("promise 超时");
}, 2000);

We change the success time to 1s.