ES6 系列之 Babel 將 Async 編譯成了什麼樣子

時間 2019-11-30

標籤 es6 系列 babel async 編譯成了什麼樣子欄目 JavaScript 简体版

原文原文鏈接

前言

本文就是簡單介紹下 Async 語法編譯後的代碼。git

Async

const fetchData = (data) => new Promise((resolve) => setTimeout(resolve, 1000, data + 1))

const fetchValue = async function () {
    var value1 = await fetchData(1);
    var value2 = await fetchData(value1);
    var value3 = await fetchData(value2);
    console.log(value3)
};

fetchValue();
// 大約 3s 後輸出 4
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Babel

咱們直接在 Babel 官網的 Try it out 粘貼上述代碼，而後查看代碼編譯成什麼樣子：github

"use strict";

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");
    });
  };
}

var fetchData = function fetchData(data) {
  return new Promise(function(resolve) {
    return setTimeout(resolve, 1000, data + 1);
  });
};

var fetchValue = (function() {
  var _ref = _asyncToGenerator(
    /*#__PURE__*/ regeneratorRuntime.mark(function _callee() {
      var value1, value2, value3;
      return regeneratorRuntime.wrap(
        function _callee$(_context) {
          while (1) {
            switch ((_context.prev = _context.next)) {
              case 0:
                _context.next = 2;
                return fetchData(1);

              case 2:
                value1 = _context.sent;
                _context.next = 5;
                return fetchData(value1);

              case 5:
                value2 = _context.sent;
                _context.next = 8;
                return fetchData(value2);

              case 8:
                value3 = _context.sent;

                console.log(value3);

              case 10:
              case "end":
                return _context.stop();
            }
          }
        },
        _callee,
        this
      );
    })
  );

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

fetchValue();
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_asyncToGenerator

regeneratorRuntime 相關的代碼咱們在《ES6 系列之 Babel 將 Generator 編譯成了什麼樣子》中已經介紹過了，此次咱們重點來看看 _asyncToGenerator 函數：babel

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");
    });
  };
}
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以上這段代碼主要是用來實現 generator 的自動執行以及返回 Promise。app

當咱們執行 fetchValue() 的時候，執行的其實就是 _asyncToGenerator 返回的這個匿名函數，在匿名函數中，咱們執行了異步

var gen = fn.apply(this, arguments);
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這一步就至關於執行 Generator 函數，舉個例子：async

function* helloWorldGenerator() {
  yield 'hello';
  yield 'world';
  return 'ending';
}

var hw = helloWorldGenerator();
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var gen = fn.apply(this, arguments) 就至關於 var hw = helloWorldGenerator();，返回的 gen 是一個具備 next()、throw()、return() 方法的對象。函數

而後咱們返回了一個 Promise 對象，在 Promise 中，咱們執行了 step("next")，step 函數中會執行：fetch

try {
  var info = gen[key](arg);
  var value = info.value;
} catch (error) {
  reject(error);
  return;
}
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step("next") 就至關於 var info = gen.next()，返回的 info 對象是一個具備 value 和 done 屬性的對象：ui

{value: Promise, done: false}
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接下來又會執行：this

if (info.done) {
  resolve(value);
} else {
  return Promise.resolve(value).then(
    function(value) {
      step("next", value);
    },
    function(err) {
      step("throw", err);
    }
  );
}
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value 此時是一個 Promise，Promise.resolve(value) 依然會返回這個 Promise，咱們給這個 Promise 添加了一個 then 函數，用於在 Promise 有結果時執行，有結果時又會執行 step("next", value)，從而使得 Generator 繼續執行，直到 info.done 爲 true，纔會 resolve(value)。

不完整但可用的代碼

(function() {
    var ContinueSentinel = {};

    var mark = function(genFun) {
        var generator = Object.create({
            next: function(arg) {
                return this._invoke("next", arg);
            }
        });
        genFun.prototype = generator;
        return genFun;
    };

    function wrap(innerFn, outerFn, self) {
        var generator = Object.create(outerFn.prototype);

        var context = {
            done: false,
            method: "next",
            next: 0,
            prev: 0,
            sent: undefined,
            abrupt: function(type, arg) {
                var record = {};
                record.type = type;
                record.arg = arg;

                return this.complete(record);
            },
            complete: function(record, afterLoc) {
                if (record.type === "return") {
                    this.rval = this.arg = record.arg;
                    this.method = "return";
                    this.next = "end";
                }

                return ContinueSentinel;
            },
            stop: function() {
                this.done = true;
                return this.rval;
            }
        };

        generator._invoke = makeInvokeMethod(innerFn, context);

        return generator;
    }

    function makeInvokeMethod(innerFn, context) {
        var state = "start";

        return function invoke(method, arg) {
            if (state === "completed") {
                return { value: undefined, done: true };
            }

            context.method = method;
            context.arg = arg;

            while (true) {
                state = "executing";

                if (context.method === "next") {
                    context.sent = context._sent = context.arg;
                }

                var record = {
                    type: "normal",
                    arg: innerFn.call(self, context)
                };

                if (record.type === "normal") {
                    state = context.done ? "completed" : "yield";

                    if (record.arg === ContinueSentinel) {
                        continue;
                    }

                    return {
                        value: record.arg,
                        done: context.done
                    };
                }
            }
        };
    }

    window.regeneratorRuntime = {};

    regeneratorRuntime.wrap = wrap;
    regeneratorRuntime.mark = mark;
})();

"use strict";

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");
        });
    };
}

var fetchData = function fetchData(data) {
    return new Promise(function(resolve) {
        return setTimeout(resolve, 1000, data + 1);
    });
};

var fetchValue = (function() {
    var _ref = _asyncToGenerator(
        /*#__PURE__*/
        regeneratorRuntime.mark(function _callee() {
            var value1, value2, value3;
            return regeneratorRuntime.wrap(
                function _callee$(_context) {
                    while (1) {
                        switch ((_context.prev = _context.next)) {
                            case 0:
                                _context.next = 2;
                                return fetchData(1);

                            case 2:
                                value1 = _context.sent;
                                _context.next = 5;
                                return fetchData(value1);

                            case 5:
                                value2 = _context.sent;
                                _context.next = 8;
                                return fetchData(value2);

                            case 8:
                                value3 = _context.sent;

                                console.log(value3);

                            case 10:
                            case "end":
                                return _context.stop();
                        }
                    }
                },
                _callee,
                this
            );
        })
    );

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

fetchValue();
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