What does _.debounce do?

基本上它会限制调用，因此如果在短时间内多次调用它，则只会调用一个实例。

你为什么要用它?

窗口之类的事件。快速连续多次调整射击大小。如果需要对新位置进行大量计算，则不希望多次执行计算。您只希望在用户完成调整大小事件时触发它。

underscore.js源代码中的描述:

返回一个函数，只要它继续被调用，就会不会被触发。函数停止后将被调用需要N毫秒。如果传入'immediate'，则触发

代码self:

_.debounce = function(func, wait, immediate) {
  var timeout, result;
  return function() {
    var context = this, args = arguments;
    var later = function() {
      timeout = null;
      if (!immediate) result = func.apply(context, args);
    };
    var callNow = immediate && !timeout;
    clearTimeout(timeout);
    timeout = setTimeout(later, wait);
    if (callNow) result = func.apply(context, args);
    return result;
  };
};

我写了一篇题为Demystifying Debounce in JavaScript的文章，在那里我准确地解释了Debounce函数是如何工作的，并包含了一个演示。

debounce函数提供了一种"节流"函数执行的方法。它们通常用于将函数绑定到快速连续触发的事件的情况。在窗口大小调整和滚动中使用debounce函数是很常见的。

无论是下划线还是其他JavaScript库，所有的debounce函数都是基于JavaScript的原生setTimeout方法构建的。因此，在深入了解debounce函数的作用之前，最好先彻底了解WindowTimers(指向MDN的链接)。

另外，你需要对作用域和闭包有一个很好的理解。尽管debounce函数的大小相对较小，但它实际上使用了一些非常高级的JavaScript概念!

话虽如此，下面是我在上面引用的帖子中解释和演示的基本debounce函数。

// Create JD Object
// ----------------
var JD = {};
// Debounce Method
// ---------------
JD.debounce = function(func, wait, immediate) {
    var timeout;
    return function() {
        var context = this,
            args = arguments;
        var later = function() {
            timeout = null;
            if ( !immediate ) {
                func.apply(context, args);
            }
        };
        var callNow = immediate && !timeout;
        clearTimeout(timeout);
        timeout = setTimeout(later, wait || 200);
        if ( callNow ) { 
            func.apply(context, args);
        }
    };
};

解释

// Create JD Object
// ----------------
/*
    It's a good idea to attach helper methods like `debounce` to your own 
    custom object. That way, you don't pollute the global space by 
    attaching methods to the `window` object and potentially run in to
    conflicts.
*/
var JD = {};
// Debounce Method
// ---------------
/*
    Return a function, that, as long as it continues to be invoked, will
    not be triggered. The function will be called after it stops being 
    called for `wait` milliseconds. If `immediate` is passed, trigger the 
    function on the leading edge, instead of the trailing.
*/
JD.debounce = function(func, wait, immediate) {
    /*
        Declare a variable named `timeout` variable that we will later use 
        to store the *timeout ID returned by the `setTimeout` function.
        *When setTimeout is called, it retuns a numeric ID. This unique ID
        can be used in conjunction with JavaScript's `clearTimeout` method 
        to prevent the code passed in the first argument of the `setTimout`
        function from being called. Note, this prevention will only occur
        if `clearTimeout` is called before the specified number of 
        milliseconds passed in the second argument of setTimeout have been
        met.
    */
    var timeout;
    /*
        Return an anomymous function that has access to the `func`
        argument of our `debounce` method through the process of closure.
    */
    return function() {
        /*
            1) Assign `this` to a variable named `context` so that the 
               `func` argument passed to our `debounce` method can be 
               called in the proper context.
            2) Assign all *arugments passed in the `func` argument of our
               `debounce` method to a variable named `args`.
            *JavaScript natively makes all arguments passed to a function
            accessible inside of the function in an array-like variable 
            named `arguments`. Assinging `arguments` to `args` combines 
            all arguments passed in the `func` argument of our `debounce` 
            method in a single variable.
        */
        var context = this,   /* 1 */
            args = arguments; /* 2 */
        /*
            Assign an anonymous function to a variable named `later`.
            This function will be passed in the first argument of the
            `setTimeout` function below.
        */
        var later = function() {
            /*      
                When the `later` function is called, remove the numeric ID 
                that was assigned to it by the `setTimeout` function.
                Note, by the time the `later` function is called, the
                `setTimeout` function will have returned a numeric ID to 
                the `timeout` variable. That numeric ID is removed by 
                assiging `null` to `timeout`.
            */
            timeout = null;
            /*
                If the boolean value passed in the `immediate` argument 
                of our `debouce` method is falsy, then invoke the 
                function passed in the `func` argument of our `debouce`
                method using JavaScript's *`apply` method.
                *The `apply` method allows you to call a function in an
                explicit context. The first argument defines what `this`
                should be. The second argument is passed as an array 
                containing all the arguments that should be passed to 
                `func` when it is called. Previously, we assigned `this` 
                to the `context` variable, and we assigned all arguments 
                passed in `func` to the `args` variable.
            */
            if ( !immediate ) {
                func.apply(context, args);
            }
        };
        /*
            If the value passed in the `immediate` argument of our 
            `debounce` method is truthy and the value assigned to `timeout`
            is falsy, then assign `true` to the `callNow` variable.
            Otherwise, assign `false` to the `callNow` variable.
        */
        var callNow = immediate && !timeout;
        /*
            As long as the event that our `debounce` method is bound to is 
            still firing within the `wait` period, remove the numerical ID  
            (returned to the `timeout` vaiable by `setTimeout`) from 
            JavaScript's execution queue. This prevents the function passed 
            in the `setTimeout` function from being invoked.
            Remember, the `debounce` method is intended for use on events
            that rapidly fire, ie: a window resize or scroll. The *first* 
            time the event fires, the `timeout` variable has been declared, 
            but no value has been assigned to it - it is `undefined`. 
            Therefore, nothing is removed from JavaScript's execution queue 
            because nothing has been placed in the queue - there is nothing 
            to clear.
            Below, the `timeout` variable is assigned the numerical ID 
            returned by the `setTimeout` function. So long as *subsequent* 
            events are fired before the `wait` is met, `timeout` will be 
            cleared, resulting in the function passed in the `setTimeout` 
            function being removed from the execution queue. As soon as the 
            `wait` is met, the function passed in the `setTimeout` function 
            will execute.
        */
        clearTimeout(timeout);
        /*
            Assign a `setTimout` function to the `timeout` variable we 
            previously declared. Pass the function assigned to the `later` 
            variable to the `setTimeout` function, along with the numerical 
            value assigned to the `wait` argument in our `debounce` method. 
            If no value is passed to the `wait` argument in our `debounce` 
            method, pass a value of 200 milliseconds to the `setTimeout` 
            function.  
        */
        timeout = setTimeout(later, wait || 200);
        /*
            Typically, you want the function passed in the `func` argument
            of our `debounce` method to execute once *after* the `wait` 
            period has been met for the event that our `debounce` method is 
            bound to (the trailing side). However, if you want the function 
            to execute once *before* the event has finished (on the leading 
            side), you can pass `true` in the `immediate` argument of our 
            `debounce` method.
            If `true` is passed in the `immediate` argument of our 
            `debounce` method, the value assigned to the `callNow` variable 
            declared above will be `true` only after the *first* time the 
            event that our `debounce` method is bound to has fired.
            After the first time the event is fired, the `timeout` variable
            will contain a falsey value. Therfore, the result of the 
            expression that gets assigned to the `callNow` variable is 
            `true` and the function passed in the `func` argument of our
            `debounce` method is exected in the line of code below.
            Every subsequent time the event that our `debounce` method is 
            bound to fires within the `wait` period, the `timeout` variable 
            holds the numerical ID returned from the `setTimout` function 
            assigned to it when the previous event was fired, and the 
            `debounce` method was executed.
            This means that for all subsequent events within the `wait`
            period, the `timeout` variable holds a truthy value, and the
            result of the expression that gets assigned to the `callNow`
            variable is `false`. Therefore, the function passed in the 
            `func` argument of our `debounce` method will not be executed.  
            Lastly, when the `wait` period is met and the `later` function
            that is passed in the `setTimeout` function executes, the 
            result is that it just assigns `null` to the `timeout` 
            variable. The `func` argument passed in our `debounce` method 
            will not be executed because the `if` condition inside the 
            `later` function fails. 
        */
        if ( callNow ) { 
            func.apply(context, args);
        }
    };
};

在超时之前保持函数的执行。这是为了避免在不需要时连续执行函数。注意，因为underscore.debounce()依赖于复杂的代码。大多数情况下，在函数中使用一个简单的"if"语句要比debounce快得多。您可以实现一个计数器，仅在每N次迭代中执行方法，或者实现一个超时，检查至少经过了一定数量的毫秒。