async 函数

小程序中使用async需要引入facebook的 runtime.js

  async foo() {
    console.log('foo')
    return 1
  },

    console.log(this.foo())

结果：

说明async是一个Promise对象。

async/await使用：

async foo() {
    console.log('foo')
    let res = await this.timeout()
    console.log(res)
  },
  timeout() {
    return new Promise((resolve, reject) => {
      setTimeout(() => {
        console.log(1)
        resolve('resolved')
      }, 1000)
    })
  },

结果：

从结果中可以看出，使用await后，会等待异步方法timeout执行完后才去执行下面的语句。

runtime.js：

/**
 * Copyright (c) 2014-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

!(function(global) {
    // "use strict";
  
    var Op = Object.prototype;
    var hasOwn = Op.hasOwnProperty;
    var undefined; // More compressible than void 0.
    var $Symbol = typeof Symbol === "function" ? Symbol : {};
    var iteratorSymbol = $Symbol.iterator || "@@iterator";
    var asyncIteratorSymbol = $Symbol.asyncIterator || "@@asyncIterator";
    var toStringTagSymbol = $Symbol.toStringTag || "@@toStringTag";
  
    var inModule = typeof module === "object";
    var runtime = global.regeneratorRuntime;
    if (runtime) {
      if (inModule) {
        // If regeneratorRuntime is defined globally and we're in a module,
        // make the exports object identical to regeneratorRuntime.
        module.exports = runtime;
      }
      // Don't bother evaluating the rest of this file if the runtime was
      // already defined globally.
      return;
    }
  
    // Define the runtime globally (as expected by generated code) as either
    // module.exports (if we're in a module) or a new, empty object.
    runtime = global.regeneratorRuntime = inModule ? module.exports : {};
  
    function wrap(innerFn, outerFn, self, tryLocsList) {
      // If outerFn provided and outerFn.prototype is a Generator, then outerFn.prototype instanceof Generator.
      var protoGenerator = outerFn && outerFn.prototype instanceof Generator ? outerFn : Generator;
      var generator = Object.create(protoGenerator.prototype);
      var context = new Context(tryLocsList || []);
  
      // The ._invoke method unifies the implementations of the .next,
      // .throw, and .return methods.
      generator._invoke = makeInvokeMethod(innerFn, self, context);
  
      return generator;
    }
    runtime.wrap = wrap;
  
    // Try/catch helper to minimize deoptimizations. Returns a completion
    // record like context.tryEntries[i].completion. This interface could
    // have been (and was previously) designed to take a closure to be
    // invoked without arguments, but in all the cases we care about we
    // already have an existing method we want to call, so there's no need
    // to create a new function object. We can even get away with assuming
    // the method takes exactly one argument, since that happens to be true
    // in every case, so we don't have to touch the arguments object. The
    // only additional allocation required is the completion record, which
    // has a stable shape and so hopefully should be cheap to allocate.
    function tryCatch(fn, obj, arg) {
      try {
        return { type: "normal", arg: fn.call(obj, arg) };
      } catch (err) {
        return { type: "throw", arg: err };
      }
    }
  
    var GenStateSuspendedStart = "suspendedStart";
    var GenStateSuspendedYield = "suspendedYield";
    var GenStateExecuting = "executing";
    var GenStateCompleted = "completed";
  
    // Returning this object from the innerFn has the same effect as
    // breaking out of the dispatch switch statement.
    var ContinueSentinel = {};
  
    // Dummy constructor functions that we use as the .constructor and
    // .constructor.prototype properties for functions that return Generator
    // objects. For full spec compliance, you may wish to configure your
    // minifier not to mangle the names of these two functions.
    function Generator() {}
    function GeneratorFunction() {}
    function GeneratorFunctionPrototype() {}
  
    // This is a polyfill for %IteratorPrototype% for environments that
    // don't natively support it.
    var IteratorPrototype = {};
    IteratorPrototype[iteratorSymbol] = function () {
      return this;
    };
  
    var getProto = Object.getPrototypeOf;
    var NativeIteratorPrototype = getProto && getProto(getProto(values([])));
    if (NativeIteratorPrototype &&
        NativeIteratorPrototype !== Op &&
        hasOwn.call(NativeIteratorPrototype, iteratorSymbol)) {
      // This environment has a native %IteratorPrototype%; use it instead
      // of the polyfill.
      IteratorPrototype = NativeIteratorPrototype;
    }
  
    var Gp = GeneratorFunctionPrototype.prototype =
      Generator.prototype = Object.create(IteratorPrototype);
    GeneratorFunction.prototype = Gp.constructor = GeneratorFunctionPrototype;
    GeneratorFunctionPrototype.constructor = GeneratorFunction;
    GeneratorFunctionPrototype[toStringTagSymbol] =
      GeneratorFunction.displayName = "GeneratorFunction";
  
    // Helper for defining the .next, .throw, and .return methods of the
    // Iterator interface in terms of a single ._invoke method.
    function defineIteratorMethods(prototype) {
      ["next", "throw", "return"].forEach(function(method) {
        prototype[method] = function(arg) {
          return this._invoke(method, arg);
        };
      });
    }
  
    runtime.isGeneratorFunction = function(genFun) {
      var ctor = typeof genFun === "function" && genFun.constructor;
      return ctor
        ? ctor === GeneratorFunction ||
          // For the native GeneratorFunction constructor, the best we can
          // do is to check its .name property.
          (ctor.displayName || ctor.name) === "GeneratorFunction"
        : false;
    };
  
    runtime.mark = function(genFun) {
      if (Object.setPrototypeOf) {
        Object.setPrototypeOf(genFun, GeneratorFunctionPrototype);
      } else {
        genFun.__proto__ = GeneratorFunctionPrototype;
        if (!(toStringTagSymbol in genFun)) {
          genFun[toStringTagSymbol] = "GeneratorFunction";
        }
      }
      genFun.prototype = Object.create(Gp);
      return genFun;
    };
  
    // Within the body of any async function, `await x` is transformed to
    // `yield regeneratorRuntime.awrap(x)`, so that the runtime can test
    // `hasOwn.call(value, "__await")` to determine if the yielded value is
    // meant to be awaited.
    runtime.awrap = function(arg) {
      return { __await: arg };
    };
  
    function AsyncIterator(generator) {
      function invoke(method, arg, resolve, reject) {
        var record = tryCatch(generator[method], generator, arg);
        if (record.type === "throw") {
          reject(record.arg);
        } else {
          var result = record.arg;
          var value = result.value;
          if (value &&
              typeof value === "object" &&
              hasOwn.call(value, "__await")) {
            return Promise.resolve(value.__await).then(function(value) {
              invoke("next", value, resolve, reject);
            }, function(err) {
              invoke("throw", err, resolve, reject);
            });
          }
  
          return Promise.resolve(value).then(function(unwrapped) {
            // When a yielded Promise is resolved, its final value becomes
            // the .value of the Promise<{value,done}> result for the
            // current iteration.
            result.value = unwrapped;
            resolve(result);
          }, function(error) {
            // If a rejected Promise was yielded, throw the rejection back
            // into the async generator function so it can be handled there.
            return invoke("throw", error, resolve, reject);
          });
        }
      }
  
      var previousPromise;
  
      function enqueue(method, arg) {
        function callInvokeWithMethodAndArg() {
          return new Promise(function(resolve, reject) {
            invoke(method, arg, resolve, reject);
          });
        }
  
        return previousPromise =
          // If enqueue has been called before, then we want to wait until
          // all previous Promises have been resolved before calling invoke,
          // so that results are always delivered in the correct order. If
          // enqueue has not been called before, then it is important to
          // call invoke immediately, without waiting on a callback to fire,
          // so that the async generator function has the opportunity to do
          // any necessary setup in a predictable way. This predictability
          // is why the Promise constructor synchronously invokes its
          // executor callback, and why async functions synchronously
          // execute code before the first await. Since we implement simple
          // async functions in terms of async generators, it is especially
          // important to get this right, even though it requires care.
          previousPromise ? previousPromise.then(
            callInvokeWithMethodAndArg,
            // Avoid propagating failures to Promises returned by later
            // invocations of the iterator.
            callInvokeWithMethodAndArg
          ) : callInvokeWithMethodAndArg();
      }
  
      // Define the unified helper method that is used to implement .next,
      // .throw, and .return (see defineIteratorMethods).
      this._invoke = enqueue;
    }
  
    defineIteratorMethods(AsyncIterator.prototype);
    AsyncIterator.prototype[asyncIteratorSymbol] = function () {
      return this;
    };
    runtime.AsyncIterator = AsyncIterator;
  
    // Note that simple async functions are implemented on top of
    // AsyncIterator objects; they just return a Promise for the value of
    // the final result produced by the iterator.
    runtime.async = function(innerFn, outerFn, self, tryLocsList) {
      var iter = new AsyncIterator(
        wrap(innerFn, outerFn, self, tryLocsList)
      );
  
      return runtime.isGeneratorFunction(outerFn)
        ? iter // If outerFn is a generator, return the full iterator.
        : iter.next().then(function(result) {
            return result.done ? result.value : iter.next();
          });
    };
  
    function makeInvokeMethod(innerFn, self, context) {
      var state = GenStateSuspendedStart;
  
      return function invoke(method, arg) {
        if (state === GenStateExecuting) {
          throw new Error("Generator is already running");
        }
  
        if (state === GenStateCompleted) {
          if (method === "throw") {
            throw arg;
          }
  
          // Be forgiving, per 25.3.3.3.3 of the spec:
          // https://people.mozilla.org/~jorendorff/es6-draft.html#sec-generatorresume
          return doneResult();
        }
  
        context.method = method;
        context.arg = arg;
  
        while (true) {
          var delegate = context.delegate;
          if (delegate) {
            var delegateResult = maybeInvokeDelegate(delegate, context);
            if (delegateResult) {
              if (delegateResult === ContinueSentinel) continue;
              return delegateResult;
            }
          }
  
          if (context.method === "next") {
            // Setting context._sent for legacy support of Babel's
            // function.sent implementation.
            context.sent = context._sent = context.arg;
  
          } else if (context.method === "throw") {
            if (state === GenStateSuspendedStart) {
              state = GenStateCompleted;
              throw context.arg;
            }
  
            context.dispatchException(context.arg);
  
          } else if (context.method === "return") {
            context.abrupt("return", context.arg);
          }
  
          state = GenStateExecuting;
  
          var record = tryCatch(innerFn, self, context);
          if (record.type === "normal") {
            // If an exception is thrown from innerFn, we leave state ===
            // GenStateExecuting and loop back for another invocation.
            state = context.done
              ? GenStateCompleted
              : GenStateSuspendedYield;
  
            if (record.arg === ContinueSentinel) {
              continue;
            }
  
            return {
              value: record.arg,
              done: context.done
            };
  
          } else if (record.type === "throw") {
            state = GenStateCompleted;
            // Dispatch the exception by looping back around to the
            // context.dispatchException(context.arg) call above.
            context.method = "throw";
            context.arg = record.arg;
          }
        }
      };
    }
  
    // Call delegate.iterator[context.method](context.arg) and handle the
    // result, either by returning a { value, done } result from the
    // delegate iterator, or by modifying context.method and context.arg,
    // setting context.delegate to null, and returning the ContinueSentinel.
    function maybeInvokeDelegate(delegate, context) {
      var method = delegate.iterator[context.method];
      if (method === undefined) {
        // A .throw or .return when the delegate iterator has no .throw
        // method always terminates the yield* loop.
        context.delegate = null;
  
        if (context.method === "throw") {
          if (delegate.iterator.return) {
            // If the delegate iterator has a return method, give it a
            // chance to clean up.
            context.method = "return";
            context.arg = undefined;
            maybeInvokeDelegate(delegate, context);
  
            if (context.method === "throw") {
              // If maybeInvokeDelegate(context) changed context.method from
              // "return" to "throw", let that override the TypeError below.
              return ContinueSentinel;
            }
          }
  
          context.method = "throw";
          context.arg = new TypeError(
            "The iterator does not provide a 'throw' method");
        }
  
        return ContinueSentinel;
      }
  
      var record = tryCatch(method, delegate.iterator, context.arg);
  
      if (record.type === "throw") {
        context.method = "throw";
        context.arg = record.arg;
        context.delegate = null;
        return ContinueSentinel;
      }
  
      var info = record.arg;
  
      if (! info) {
        context.method = "throw";
        context.arg = new TypeError("iterator result is not an object");
        context.delegate = null;
        return ContinueSentinel;
      }
  
      if (info.done) {
        // Assign the result of the finished delegate to the temporary
        // variable specified by delegate.resultName (see delegateYield).
        context[delegate.resultName] = info.value;
  
        // Resume execution at the desired location (see delegateYield).
        context.next = delegate.nextLoc;
  
        // If context.method was "throw" but the delegate handled the
        // exception, let the outer generator proceed normally. If
        // context.method was "next", forget context.arg since it has been
        // "consumed" by the delegate iterator. If context.method was
        // "return", allow the original .return call to continue in the
        // outer generator.
        if (context.method !== "return") {
          context.method = "next";
          context.arg = undefined;
        }
  
      } else {
        // Re-yield the result returned by the delegate method.
        return info;
      }
  
      // The delegate iterator is finished, so forget it and continue with
      // the outer generator.
      context.delegate = null;
      return ContinueSentinel;
    }
  
    // Define Generator.prototype.{next,throw,return} in terms of the
    // unified ._invoke helper method.
    defineIteratorMethods(Gp);
  
    Gp[toStringTagSymbol] = "Generator";
  
    // A Generator should always return itself as the iterator object when the
    // @@iterator function is called on it. Some browsers' implementations of the
    // iterator prototype chain incorrectly implement this, causing the Generator
    // object to not be returned from this call. This ensures that doesn't happen.
    // See https://github.com/facebook/regenerator/issues/274 for more details.
    Gp[iteratorSymbol] = function() {
      return this;
    };
  
    Gp.toString = function() {
      return "[object Generator]";
    };
  
    function pushTryEntry(locs) {
      var entry = { tryLoc: locs[0] };
  
      if (1 in locs) {
        entry.catchLoc = locs[1];
      }
  
      if (2 in locs) {
        entry.finallyLoc = locs[2];
        entry.afterLoc = locs[3];
      }
  
      this.tryEntries.push(entry);
    }
  
    function resetTryEntry(entry) {
      var record = entry.completion || {};
      record.type = "normal";
      delete record.arg;
      entry.completion = record;
    }
  
    function Context(tryLocsList) {
      // The root entry object (effectively a try statement without a catch
      // or a finally block) gives us a place to store values thrown from
      // locations where there is no enclosing try statement.
      this.tryEntries = [{ tryLoc: "root" }];
      tryLocsList.forEach(pushTryEntry, this);
      this.reset(true);
    }
  
    runtime.keys = function(object) {
      var keys = [];
      for (var key in object) {
        keys.push(key);
      }
      keys.reverse();
  
      // Rather than returning an object with a next method, we keep
      // things simple and return the next function itself.
      return function next() {
        while (keys.length) {
          var key = keys.pop();
          if (key in object) {
            next.value = key;
            next.done = false;
            return next;
          }
        }
  
        // To avoid creating an additional object, we just hang the .value
        // and .done properties off the next function object itself. This
        // also ensures that the minifier will not anonymize the function.
        next.done = true;
        return next;
      };
    };
  
    function values(iterable) {
      if (iterable) {
        var iteratorMethod = iterable[iteratorSymbol];
        if (iteratorMethod) {
          return iteratorMethod.call(iterable);
        }
  
        if (typeof iterable.next === "function") {
          return iterable;
        }
  
        if (!isNaN(iterable.length)) {
          var i = -1, next = function next() {
            while (++i < iterable.length) {
              if (hasOwn.call(iterable, i)) {
                next.value = iterable[i];
                next.done = false;
                return next;
              }
            }
  
            next.value = undefined;
            next.done = true;
  
            return next;
          };
  
          return next.next = next;
        }
      }
  
      // Return an iterator with no values.
      return { next: doneResult };
    }
    runtime.values = values;
  
    function doneResult() {
      return { value: undefined, done: true };
    }
  
    Context.prototype = {
      constructor: Context,
  
      reset: function(skipTempReset) {
        this.prev = 0;
        this.next = 0;
        // Resetting context._sent for legacy support of Babel's
        // function.sent implementation.
        this.sent = this._sent = undefined;
        this.done = false;
        this.delegate = null;
  
        this.method = "next";
        this.arg = undefined;
  
        this.tryEntries.forEach(resetTryEntry);
  
        if (!skipTempReset) {
          for (var name in this) {
            // Not sure about the optimal order of these conditions:
            if (name.charAt(0) === "t" &&
                hasOwn.call(this, name) &&
                !isNaN(+name.slice(1))) {
              this[name] = undefined;
            }
          }
        }
      },
  
      stop: function() {
        this.done = true;
  
        var rootEntry = this.tryEntries[0];
        var rootRecord = rootEntry.completion;
        if (rootRecord.type === "throw") {
          throw rootRecord.arg;
        }
  
        return this.rval;
      },
  
      dispatchException: function(exception) {
        if (this.done) {
          throw exception;
        }
  
        var context = this;
        function handle(loc, caught) {
          record.type = "throw";
          record.arg = exception;
          context.next = loc;
  
          if (caught) {
            // If the dispatched exception was caught by a catch block,
            // then let that catch block handle the exception normally.
            context.method = "next";
            context.arg = undefined;
          }
  
          return !! caught;
        }
  
        for (var i = this.tryEntries.length - 1; i >= 0; --i) {
          var entry = this.tryEntries[i];
          var record = entry.completion;
  
          if (entry.tryLoc === "root") {
            // Exception thrown outside of any try block that could handle
            // it, so set the completion value of the entire function to
            // throw the exception.
            return handle("end");
          }
  
          if (entry.tryLoc <= this.prev) {
            var hasCatch = hasOwn.call(entry, "catchLoc");
            var hasFinally = hasOwn.call(entry, "finallyLoc");
  
            if (hasCatch && hasFinally) {
              if (this.prev < entry.catchLoc) {
                return handle(entry.catchLoc, true);
              } else if (this.prev < entry.finallyLoc) {
                return handle(entry.finallyLoc);
              }
  
            } else if (hasCatch) {
              if (this.prev < entry.catchLoc) {
                return handle(entry.catchLoc, true);
              }
  
            } else if (hasFinally) {
              if (this.prev < entry.finallyLoc) {
                return handle(entry.finallyLoc);
              }
  
            } else {
              throw new Error("try statement without catch or finally");
            }
          }
        }
      },
  
      abrupt: function(type, arg) {
        for (var i = this.tryEntries.length - 1; i >= 0; --i) {
          var entry = this.tryEntries[i];
          if (entry.tryLoc <= this.prev &&
              hasOwn.call(entry, "finallyLoc") &&
              this.prev < entry.finallyLoc) {
            var finallyEntry = entry;
            break;
          }
        }
  
        if (finallyEntry &&
            (type === "break" ||
             type === "continue") &&
            finallyEntry.tryLoc <= arg &&
            arg <= finallyEntry.finallyLoc) {
          // Ignore the finally entry if control is not jumping to a
          // location outside the try/catch block.
          finallyEntry = null;
        }
  
        var record = finallyEntry ? finallyEntry.completion : {};
        record.type = type;
        record.arg = arg;
  
        if (finallyEntry) {
          this.method = "next";
          this.next = finallyEntry.finallyLoc;
          return ContinueSentinel;
        }
  
        return this.complete(record);
      },
  
      complete: function(record, afterLoc) {
        if (record.type === "throw") {
          throw record.arg;
        }
  
        if (record.type === "break" ||
            record.type === "continue") {
          this.next = record.arg;
        } else if (record.type === "return") {
          this.rval = this.arg = record.arg;
          this.method = "return";
          this.next = "end";
        } else if (record.type === "normal" && afterLoc) {
          this.next = afterLoc;
        }
  
        return ContinueSentinel;
      },
  
      finish: function(finallyLoc) {
        for (var i = this.tryEntries.length - 1; i >= 0; --i) {
          var entry = this.tryEntries[i];
          if (entry.finallyLoc === finallyLoc) {
            this.complete(entry.completion, entry.afterLoc);
            resetTryEntry(entry);
            return ContinueSentinel;
          }
        }
      },
  
      "catch": function(tryLoc) {
        for (var i = this.tryEntries.length - 1; i >= 0; --i) {
          var entry = this.tryEntries[i];
          if (entry.tryLoc === tryLoc) {
            var record = entry.completion;
            if (record.type === "throw") {
              var thrown = record.arg;
              resetTryEntry(entry);
            }
            return thrown;
          }
        }
  
        // The context.catch method must only be called with a location
        // argument that corresponds to a known catch block.
        throw new Error("illegal catch attempt");
      },
  
      delegateYield: function(iterable, resultName, nextLoc) {
        this.delegate = {
          iterator: values(iterable),
          resultName: resultName,
          nextLoc: nextLoc
        };
  
        if (this.method === "next") {
          // Deliberately forget the last sent value so that we don't
          // accidentally pass it on to the delegate.
          this.arg = undefined;
        }
  
        return ContinueSentinel;
      }
    };
  })(
    // In sloppy mode, unbound `this` refers to the global object, fallback to
    // Function constructor if we're in global strict mode. That is sadly a form
    // of indirect eval which violates Content Security Policy.
    (function() { return this })() || Function("return this")()
  );