Javascript ||操作者 [英] Javascript || operator

问题描述

我正在使用一个大型JS库在canvas中执行某些绘图操作。检查库代码（以进行适应性更改），我遇到了以某种方式使用的||运算符，该方式当然不应该计算为布尔值。这是否意味着这是一个不同的运营商，还是我错过了一些明显的东西？示例如下：

I am using a large JS library to perform certain drawing operations in canvas. Reviewing the library code (to make accommodating changes), I have ran into the '||' operator being used in a fashion which certainly should not evaluate to Boolean. Does this mean that this is a different operator or am I missing something obvious? An example follows:

var $time = Date.now || function(){
return +new Date;
};

推荐答案

|| operator 计算到第一个操作数，如果可以的话转换为true或否则转换为最后一个操作数。所以在你的例子中， $ time 将是 Date.now 如果它存在或者声明的函数不存在。

The || operator evaluates to the first operand if it can be converted to true or the last operand otherwise. So in your example $time will be Date.now if it exists or the declared function otherwise.

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