javascript - 一般 JavaScript 语法问题

Question

在下面的代码中，为什么两个方法(increment 和 print)都列在 return 中？为什么不能只使用 return counter++？另外，返回 console.log 是什么意思？

function create() {
  var counter = 0;
  return {
    increment: function() {
      counter++;
    },
    print: function() {
      console.log(counter);
    }
  }
}

谢谢!

Answer 1

这返回的或多或少是一个计数器，如果我们重命名最顶层的函数，它应该更有意义。

它有什么作用？让我们添加一些评论

function Counter() { // a function, nothing special here
  var counter = 0; // a variable that's local to the function Counter
  return { // return an object literal {}
    // which has a property named 'increment'
    increment: function() { // that's a function AND a closure
      counter++; // and therefore still has access to the variable 'counter' inside of Counter
    },
    print: function() { // another function
      console.log(counter); // this logs to the console in most web browser
                            // the console object was introduces by Firebug 
                            // and effort is made being to standardize it
    }
  }
}

示例用法:

var a = Counter(); // you don't need the new keyword here sinc in this case
                   // there's no difference because the implicit return overwrites
                   // the normal constructor behavior of returning 'this'
a.increment();
a.print(); // 1
var b = Counter();
b.print(); // 0

请注意函数内部的变量counter 无法从外部访问，因此它是只读，您只能通过使用closure 来实现这种效果。 .