async 与 await 原理

async 会导致函数返回值变化为Promise， 但如果 async 函数内没有await，虽然结果返回的是 promise但是，执行还是在同步执行

原代码

let a = 0;

let yideng = async () => {
    a = a + (await 10);
    console.log(a);
};

yideng();

console.log(++a);

babel 编译后代码

'use strict';

function asyncGeneratorStep(gen, resolve, reject, _next, _throw, key, arg) {
    try {
        var info = gen[key](arg);
        var value = info.value;
    } catch (error) {
        reject(error);
        return;
    }
    if (info.done) {
        resolve(value);
    } else {
        Promise.resolve(value).then(_next, _throw);
    }
}

function _asyncToGenerator(fn) {
    return function () {
        var self = this,
            args = arguments;
        return new Promise(function (resolve, reject) {
            var gen = fn.apply(self, args);
            function _next(value) {
                asyncGeneratorStep(gen, resolve, reject, _next, _throw, 'next', value);
            }
            function _throw(err) {
                asyncGeneratorStep(gen, resolve, reject, _next, _throw, 'throw', err);
            }
            _next(undefined);
        });
    };
}

var a = 0;

var yideng = /*#__PURE__*/ (function () {
    var _ref = _asyncToGenerator(
        /*#__PURE__*/ regeneratorRuntime.mark(function _callee() {
            return regeneratorRuntime.wrap(function _callee$(_context) {
                while (1) {
                    switch ((_context.prev = _context.next)) {
                        case 0:
                            _context.t0 = a;
                            _context.next = 3;
                            return 10;

                        case 3:
                            _context.t1 = _context.sent;
                            a = _context.t0 + _context.t1;
                            console.log(a);

                        case 6:
                        case 'end':
                            return _context.stop();
                    }
                }
            }, _callee);
        })
    );

    return function yideng() {
        return _ref.apply(this, arguments);
    };
})();

yideng();
console.log(++a);