Closure Compiler为什么不缩短这个？ [英] Why does Closure Compiler not shorten this?

问题描述

我不确定这只是一个错误还是一个预期的功能。

I'm not sure whether this is just a bug or an intended feature.

基本上，我有这个小功能（我现在看到 end 在这里是蓝色的，但是这很好用，如果我将它重命名为其他东西我还有问题）：

Basically, I have this tiny function (I now see end is colored blue here but this works just fine, if I rename it to something else I still have the issue):

function f(a, b) {
    var start = Math.min(a, b);
    var end = Math.max(a, b);

    tb.selectionStart = start;
    tb.selectionEnd = end;
};

关闭编译时，我得到：

function f(a,b){var c=Math.max(a,b);tb.selectionStart=Math.min(a,b);tb.selectionEnd=c};

但是，为什么 selectionStart 设置为 Math.min 直接，而 selecitonEnd 设置为变量（ c ），首先宣布？是不是更短 tb.selectionEnd = Math.max（a，b）？

However, why is selectionStart set to Math.min directly, whilst selecitonEnd is set to a variable (c), which is declared first? Isn't it shorter to do tb.selectionEnd=Math.max(a,b)?

任何想法非常感谢。

推荐答案

编辑：此链接中有官方回答：
https： //web.archive.org/web/20151226143155/http://code.google.com/p/closure-compiler/issues/detail?id=410

THERE IS AN "OFFICIAL" ANSWER IN THIS LINK: https://web.archive.org/web/20151226143155/http://code.google.com/p/closure-compiler/issues/detail?id=410

我认为可以内联对变量的赋值，紧接着使用该变量。但是，如果中间有任何声明无法证明没有副作用，那么编译器就不会内联它。

I think an assignment to a variable, followed immediately by usage of that variable, can be inlined. However, if there is any statement in between that cannot be proven to be free of side-effects, then the compiler won't inline it.

在你的情况下，赋值变量start仅与赋值语句end的start的使用分开。但是，这个语句没有副作用，因为Math.max是一个内部函数，编译器知道它是无副作用的。

In your case, assignment to variable "start" is separated from the usage of "start" only by the assignment statement to "end". However, this statement is free of side-effects since Math.max is an internal function and the compiler knows that it is side-effect-free.

然而，在你的case，对变量end的赋值通过语句与该变量的用法分开，该语句是对属性的start赋值。现在，我相信编译器并不认为仅仅分配属性总是没有副作用;这是因为某些属性在分配时实际上会导致不同的行为，或者更改全局状态（例如RegExp）。在某些系统中，属性分配实际上会触发某些特定于系统的功能（例如硬件接口），这些功能可能会反过来包含副作用。

However, in your case, assignment to variable "end" is separated from the usage of that variable by a statement, which is an assignment of "start" to a property. Now, I believe that the compiler does not assume that merely assigning to a property is always side-effect-free; that is because some properties, when assigned, actually cause different behavior, or change global state (such as RegExp). In some systems, property assignments actually trigger certain system-specific features (e.g. hardware interface) that may in-turn contain side-effects.

这就是为什么，有时，何时你有这样的代码：

That is why, sometimes, when you have code like this:

foo.bar = 1;
foo.bar = 2;
foo.bar = 3;

编译器不会消除前两个语句，因为赋值给bar可能会产生副作用。

The compiler won't eliminate the first two statements since assignment to "bar" may have side effects.

因此，在您的问题中，变量end无法内联，因为语句 tb.selectionStart = start; 可能有副作用（可能仅在奇怪的情况下）。

So, in your question, the variable "end" cannot be inlined because the statement tb.selectionStart = start; may have side effects (perhaps only in wierd cases).

如果你使tb成为一个局部变量，或者编译器完全控制了（例如一个简单的对象： var tb = {}; ），然后你会发现编译器内联所有的赋值就好了。

If you make "tb" a local variable, or something that the compiler has complete control of (e.g. a simple object: var tb = {};), then you'll find that the compiler inlines all of the assignments just fine.

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