匿名函数和变量范围

Question

我正在阅读一篇关于 requestAnimationFrame here 的文章，我意识到我在跟踪变量的范围和持久性时遇到了问题。以下代码稍作修改：

(function() {

    //Note: lastTime is defined up here.
    var lastTime = 0;

    var vendors = ['ms', 'moz', 'webkit', 'o'];

    //Okay, so they're trying to set window.rAF to the appropriate thing based on browser
    for(var x = 0; x < vendors.length && !window.requestAnimationFrame; ++x) {
        window.requestAnimationFrame = window[vendors[x]+'RequestAnimationFrame'];
        window.cancelRequestAnimationFrame = window[vendors[x]+
          'CancelRequestAnimationFrame'];
    }

    //...and if that doesn't work, turn it into a setTimeout
    if (!window.requestAnimationFrame)
        window.requestAnimationFrame = function(callback, element) {

            //I guess this is a way to make sure that the wait time
            //between renders is consistent

            var currTime = new Date().getTime();
            var timeToCall = Math.max(0, 16 - (currTime - lastTime));
            var id = window.setTimeout(function() { callback(currTime + timeToCall); }, 
              timeToCall);

            lastTime = currTime + timeToCall;

            //Wait. They just assigned lastTime a value.
            //Why is this going to persist between calls to window.rAF?
            //The variable lastTime was defined inside this function, not in the window.

            return id;
        };

    if (!window.cancelAnimationFrame)
        window.cancelAnimationFrame = function(id) {
            clearTimeout(id);
        };
}())

我的猜测是它与放在括号内的函数有关，但是如何？使用这种编码风格可以完成什么以及我可以期待什么其他效果？这是我应该开始更经常使用的东西吗？如果是，什么时候开始？

Answer 1

这里的变量lastTime是通过closure捕获的。这意味着它在其定义的函数范围之外保持活动状态。

每当匿名函数体引用其范围之外的变量时，都会创建闭包。闭包在 JavaScript 中非常有用，因为它们允许您在不全局公开状态的情况下维护状态。

举个简单的例子，

function foo() {
    var count = 0;

    setInterval(function bar() {
        console.log(count++);
    }, 100);
}

通过在此处关闭 count 变量，我可以在 setInterval 中使用它，而无需像其他情况下那样将 count 暴露在全局范围内。