JavaScript 中的变量有多昂贵? [英] How expensive are variables in JavaScript?

问题描述

局部变量 (var v)、全局变量 (window.v) 和跨全局变量 (parent.v) 的开销有多大) 在 JavaScript 中，在主要浏览器中?有没有人对此进行过任何好的测试?

How expensive are local variables (var v), global variables (window.v) and cross-global variables (parent.v) in JavaScript, in the major browsers? Has anyone performed any good tests on this one?

推荐答案

忽略解释器/解析器的优点和缺点，重要的是运行时必须查看作用域链的各个部分.

Ignoring interpreter/parser pros and cons, all that matters is how much the runtime has to look at parts of the scope chain.

考虑这个简单的例子:

foo = 42;         // implicitly window.foo; cost: 2
var bar = 3;      // cost: 1

function woot(a) {
    a;            // cost: 1
    bar;          // cost: 2
    foo;          // cost: 3

    var other = 9;// cost: 1

    other;        // cost: 1

    a.something;  // cost: 2
    foo.win.fail.arg.func(a.something).thing = 0; // cost: 8 + 2
}

woot(bar);        // cost: 2 + 1

请记住，函数与变量共享相同的命名空间(我认为)，并且在解析方面就像变量一样.

Remember that functions share the same namespace as variables (I think) and act like variables with regards to resolution.

使用 foo['bar'] 而不是 foo.bar 没有额外的成本，除了可能由 Javscript 引擎优化(尽管如果你正在优化后者应该很容易优化，如果内容是文字的话).

There is no additional cost to using foo['bar'] instead of foo.bar, except perhaps optimizations by the Javscript engine (though if you're optimizing the latter it should be easy to optimize the former if the contents are literal).

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