&&的这两种用法之间有什么区别?逻辑运算符?

问题描述

&&"的这些各种用法之间有什么区别?逻辑运算符?

What is the difference between these various uses of the "&&" logical operator?

来自Oliver Steele的Functional.js库.第4行，"args.length&& arg":

From Oliver Steele's Functional.js library.Line 4, "args.length && arg":

0 Function.prototype.partial = function(){
1    var fn = this, args = Array.prototype.slice.call(arguments);
2    return function(){
3      var arg = 0;
4      for ( var i = 0; i < args.length && arg < arguments.length; i++ )
5        if ( args[i] === undefined )
6          args[i] = arguments[arg++];
7      return fn.apply(this, args);
8    };
9  };

来自 bootstrap.js .第11行下面的'hover'&&this.$ element":

From bootstrap.js.Line 11 below, "'hover' && this.$element":

1 var Carousel = function (element, options) {
2    this.$element    = $(element).on('keydown.bs.carousel', $.proxy(this.keydown, this))
3    this.$indicators = this.$element.find('.carousel-indicators')
4    this.options     = options
5    this.paused      =
6    this.sliding     =
7    this.interval    =
8    this.$active     =
9    this.$items      = null
10
11   this.options.pause == 'hover' && this.$element
12     .on('mouseenter.bs.carousel', $.proxy(this.pause, this))
13     .on('mouseleave.bs.carousel', $.proxy(this.cycle, this))
14 }

为什么不只在第一个示例中使用+算术运算符?

Also why not just use the + arithmetic operator in the first example?

这是我在bootstrap.js的相同轮播"部分中遇到问题时遇到的另一个例子:

Here is yet another example that I'm having trouble grokking, from the same Carousel section of bootstrap.js:

this.options.interval
      && !this.paused
      && (this.interval = setInterval($.proxy(this.next, this), this.options.interval))

推荐答案

好吧，它到处都是相同的运算符，但是程序员将其用于不同的目的.

Well, it's the same operator everywhere, but being used for different purposes by the programmer.

第一个代码示例根本不执行args.length && arg，而是执行了(i < args.length) && (arg < arguments.length)的等效功能，因为&&的优先级低于<.这意味着，循环直到i等于或超过自变量计数或arg达到或超过自变量计数为止".您可以看到+因此在这种情况下不是等效的.

The first code example is not doing args.length && arg at all, but is rather doing the equivalent of (i < args.length) && (arg < arguments.length) because && has lower precedence than <. It means, "loop until either i is at or surpasses the argument count or arg is at or surpasses the argument count". You can see that + is therefore not equivalent in this context.

其他代码示例都利用以下事实:当逻辑和运算符(&&)的左操作数为false时，就会短路"，即仅在以下情况下才对右侧的表达式求值:

The other code examples are all exploiting the fact that the logical-and operator (&&) "short-circuits" when its left-hand operand is false, i.e. it only evaluates the expression on the right if the one on the left is true.

因此，this.options.pause == 'hover' && this.$element.on(...是仅在将suspend选项设置为"hover"时附加事件侦听器的简便方法(因为如果未设置，则左侧表达式将为false，而on函数将不会被调用.)

So, this.options.pause == 'hover' && this.$element.on(... is a short-hand way of attaching the event listeners only if the pause option is set to 'hover' (because if it's not, the left-hand expression will be false, and the on functions won't be called).

类似地，如果所需的时间间隔为"true"(我想这里通常非零，但也未定义，等等)，您发现的最后一个示例仅调用setInterval(并将结果分配给this.interval).并且轮播没有暂停.

Similarly, the final example you found only calls setInterval (and assigns the result to this.interval) if the desired interval is "true" (I imagine generally non-zero here, but also not undefined, etc.) and the carousel is not paused.

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