ts.transpile does't handle async/await correctly

[weswigham]

It emits the __awaiter and transforms await statements into yield statements, but functions marked async aren't transformed to wrappers around an __awaiter'd generator.

For example,

var a = async () => await Promise.resolve(2);

yields

var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, Promise, generator) {
    return new Promise(function (resolve, reject) {
        generator = generator.call(thisArg, _arguments);
        function cast(value) { return value instanceof Promise && value.constructor === Promise ? value : new Promise(function (resolve) { resolve(value); }); }
        function onfulfill(value) { try { step("next", value); } catch (e) { reject(e); } }
        function onreject(value) { try { step("throw", value); } catch (e) { reject(e); } }
        function step(verb, value) {
            var result = generator[verb](value);
            result.done ? resolve(result.value) : cast(result.value).then(onfulfill, onreject);
        }
        step("next", void 0);
    });
};
var a = function () { return yield Promise.resolve(2); };

when it should probably be

var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, Promise, generator) {
    return new Promise(function (resolve, reject) {
        generator = generator.call(thisArg, _arguments);
        function cast(value) { return value instanceof Promise && value.constructor === Promise ? value : new Promise(function (resolve) { resolve(value); }); }
        function onfulfill(value) { try { step("next", value); } catch (e) { reject(e); } }
        function onreject(value) { try { step("throw", value); } catch (e) { reject(e); } }
        function step(verb, value) {
            var result = generator[verb](value);
            result.done ? resolve(result.value) : cast(result.value).then(onfulfill, onreject);
        }
        step("next", void 0);
    });
};
var a = function() { return __awaiter(void 0, void 0, Promise, function*() { return yield Promise.resolve(2); }); };

[vladima]

async functions are supported only in ES6 and by default transpile uses ES5. Everything should work fine if you pass ScriptTarget,ES6 through compiler options.

// test.js
var ts = require('typescript');
console.log(ts.transpile("var a = async () => await Promise.resolve(2);", { target: ts.ScriptTarget.ES6 }))

C:\Sources\1>npm install typescript@next
[email protected] node_modules\typescript

C:\Sources\1>node test.js
var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, Promise, generator) {
    return new Promise(function (resolve, reject) {
        generator = generator.call(thisArg, _arguments);
        function cast(value) { return value instanceof Promise && value.constructor === Promise ? value : new Promise(function (resolve) { resolve(value); }); }
        function onfulfill(value) { try { step("next", value); } catch (e) { reject(e); } }
        function onreject(value) { try { step("throw", value); } catch (e) { reject(e); } }
        function step(verb, value) {
            var result = generator[verb](value);
            result.done ? resolve(result.value) : cast(result.value).then(onfulfill, onreject);
        }
        step("next", void 0);
    });
};
var a = () => __awaiter(this, void 0, Promise, function* () { return yield Promise.resolve(2); });