async to generater

async-test.js

async function a(n) {
    let c = 0;
    let s = 0;
    while (c < n) {
        c += 1;
        let x = await c;  // c 也可以是其他 promise
        s += x;
        console.log(s);
    }
    return s
}

async function b() {
    let v1 = await a(1);
    let v2 = await a(2);
    let v3 = await a(3);
    let v4 = await a(4);
    let v5 = await a(5);

    let x = v1 + v2 + v3 + v4 + v5;
    return x;
}

b().then(r => console.log(r));

async-to-generater.js

function _asyncToGenerator(fn) {
    return function () {
        var gen = fn.apply(this, arguments);
        return new Promise(function (resolve, reject) {
            function step(key, arg) {
                try {
                    var info = gen[key](arg);
                    var value = info.value;
                } catch (error) {
                    reject(error);
                    return;
                }
                if (info.done) {
                    resolve(value);
                } else {
                    return Promise.resolve(value).then(function (value) {
                        step("next", value);
                    }, function (err) {
                        step("throw", err);
                    });
                }
            }
            return step("next");
        });
    };
}


function* a(n) {
    let c = 0;
    let s = 0;
    while (c < n) {
        c += 1;
        let x = yield c;   // c 也可以是其他 promise()
        s += x;
        console.log(s);
    }
    return s
}

function* b() {
    let v1 = yield ag(1);
    let v2 = yield ag(2);
    let v3 = yield ag(3);
    let v4 = yield ag(4);
    let v5 = yield ag(5);

    let x = v1 + v2 + v3 + v4 + v5;
    return x;
}


let ag = (() => {
    let aG = _asyncToGenerator(a);
    return function (n) {
        let ag1 = aG.apply(this, arguments);
        return ag1;
    }
})();

let bg = (() => {
    let bG = _asyncToGenerator(b);
    return function () {
        let ag1 = bG.apply();
        return ag1;
    }
})();


bg().then(r => {
    console.log(r)
});

将 ES2017 中的 async/await 语法通过 babel 转换为 ES5 , 源码对比.

有助于理解 yield 生成器.

yarn add babel-preset-es2017 -D

.babelrc :

{
    "presets": [
        "es2017"
    ]
}