如何使用javascript中的闭包访问函数内另一个作用域中的变量？

Question

我有以下功能makeStopwatch,我正在努力更好地理解javascript闭包:

var makeStopwatch = function() {
  var elapsed = 0;
  var stopwatch = function() {
    return elapsed;
  };
  var increase = function() {
    elapsed++;
  };

  setInterval(increase, 1000);
  return stopwatch;
};

var stopwatch1 = makeStopwatch();
var stopwatch2 = makeStopwatch();

console.log(stopwatch1());
console.log(stopwatch2());

当我console.log打电话给我时stopwatch1,stopwatch2我0每次都会回来.

据我了解的预期功能makeStopwatch的变量elapsed将0通过内部函数若返回stopwatch.内部函数increase递增变量elapsed.然后在延迟1秒后setInterval拨打电话increase.最后,stopwatch这次使用预期的更新值再次返回1.

但是,这并不因为里面工作makeStopwatch,内部stopwatch,increase和setInterval功能都在相互独立的范围？

如何根据我的理解修改它以使其elapsed增加并且该值被关闭并保存,以便当我分配makeStopwatch给变量stopwatch1并调用stopwatch1更新的值时返回？

Answer 1

var makeStopwatch = function() {
  var elapsed = 0;

  // THIS stopwatch function is referenced later
  var stopwatch = function() {
    return elapsed;
  };

  var increase = function() {
    elapsed++;
  };
  // This setInterval will continue running and calling the increase function.
  // we do not maintain access to it.
  setInterval(increase, 1000);

  // THIS returns the stopwatch function reference earlier.  The only way
  // we can interact with the closure variables are through this function.
  return stopwatch;
};

var stopwatch1 = makeStopwatch();
// This runs the makeStopwatch function.  That function *RETURNS* the
// inner stopwatch function that I emphasized above.

console.log(stopwatch1());
// stopwatch1 is a reference to the inner stopwatch function.  We no longer
// have access to the elapsed variable or the function increase.  However
// the `setInterval` that is calling `increase` is still running.  So every
// 1000ms (1 second) we increment elapsed by 1.

因此，如果将上述所有代码放入控制台，然后console.log(stopwatch1())偶尔调用，它将 console.log 自我们创建秒表以来的秒数。