javascript - 基本的 JavaScript 内嵌函数解释

Question

我正在尝试通过使用 JavaScript Koans 测试它来学习 JavaScript。我遇到过这段我很难真正理解的代码，也许有人可以教育我？

代码:

  it("should use lexical scoping to synthesise functions", function () {

    function makeMysteryFunction(makerValue)
    {
      var newFunction = function doMysteriousThing(param)
      {
        return makerValue + param;
      };
      return newFunction;
    }

    var mysteryFunction3 = makeMysteryFunction(3);
    var mysteryFunction5 = makeMysteryFunction(5);

    expect(mysteryFunction3(10) + mysteryFunction5(5)).toBe(FILL_ME_IN);
  });

因此“FILL_ME_IN”必须为 23 才能通过此测试。但有人能解释一下为什么吗？

Answer 1

makeMysteryFunction 返回的函数是一个 closure 的例子.如果您要像这样重写代码，它可能更具可读性:

it("should use lexical scoping to synthesise functions", function () {

    // This function takes a number and returns a function
    function getAddingFunction(firstNumber){
        // The function it returns takes a *second* number and returns the sum of both numbers.
        // The function returned is an example of a 'closure' - https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Closures
        // This means it remembers the scope in which it was created and, therefore, the `firstNumber` variable.
        return function addNumbers(secondNumber){
            // The function remembers firstNumber from its creation, and takes secondNumber upon use.
            return firstNumber + secondNumber;
        };
    }

    var addTo3 = getAddingFunction(3); // Returns a function which remembers firstNumber = 3
    var addTo5 = getAddingFunction(5); // Returns a function which remembers firstNumber = 5

    expect(addTo3(10) + addTo5(5)).toBe(FILL_ME_IN);
});