为什么arr = []比arr = new Array更快？ [英] Why is arr = [] faster than arr = new Array?
问题描述
我运行此代码并获得以下结果。我很想知道为什么 [] 更快？
I ran this code and got the below result. I curious to know why [] is faster?
console.time('using[]')
for(var i=0; i<200000; i++){var arr = []};
console.timeEnd('using[]')
console.time('using new')
for(var i=0; i<200000; i++){var arr = new Array};
console.timeEnd('using new')
- 使用
[]：299ms - 使用
new：363ms - using
[]: 299ms - using
new: 363ms
感谢 Raynos 这里是这个代码的基准等等定义变量的可能方法。
Thanks to Raynos here is a benchmark of this code and some more possible way to define a variable.
推荐答案
进一步扩展以前的答案......
Further expanding on previous answers...
从一般编制者的角度来看，无视特定于VM的优化：
From a general compilers perspective and disregarding VM-specific optimizations:
首先，我们通过词法分析阶段对代码进行标记。
First, we go through the lexical analysis phase where we tokenize the code.
举例来说，可以生成以下标记：
By way of example, the following tokens may be produced:
[]: ARRAY_INIT
[1]: ARRAY_INIT (NUMBER)
[1, foo]: ARRAY_INIT (NUMBER, IDENTIFIER)
new Array: NEW, IDENTIFIER
new Array(): NEW, IDENTIFIER, CALL
new Array(5): NEW, IDENTIFIER, CALL (NUMBER)
new Array(5,4): NEW, IDENTIFIER, CALL (NUMBER, NUMBER)
new Array(5, foo): NEW, IDENTIFIER, CALL (NUMBER, IDENTIFIER)
希望这应该为您提供足够的可视化，以便您了解更多（或更少）处理是必需的。
Hopefully this should provide you a sufficient visualization so you can understand how much more (or less) processing is required.
-
基于上述代币，我们知道ARRAY_INIT将始终生成一个数组。因此，我们只需创建一个数组并填充它。至于歧义，词法分析阶段已经将ARRAY_INIT与对象属性访问器（例如
obj [foo]）或字符串/正则表达式文字中的括号区分开（例如foo ] bar或/ [] /）
Based on the above tokens, we know as a fact ARRAY_INIT will always produce an array. We therefore simply create an array and populate it. As far as ambiguity, the lexical analysis stage has already distinguished ARRAY_INIT from an object property accessor (e.g.
obj[foo]) or brackets inside strings/regex literals (e.g. "foo[]bar" or /[]/)
这是微不足道的，但我们还有更多的令牌，带有 new Array 。此外，还不完全清楚我们只是想创建一个数组。我们看到新令牌，但新令牌是什么？然后我们看到IDENTIFIER标记，表示我们想要一个新的数组，但JavaScript VM通常不区分IDENTIFIER标记和本机全局对象的标记。因此......
This is miniscule, but we also have more tokens with new Array. Furthermore, it's not entirely clear yet that we simply want to create an array. We see the "new" token, but "new" what? We then see the IDENTIFIER token which signifies we want a new "Array," but JavaScript VM's generally do not distinguish an IDENTIFIER token and tokens for "native global objects." Therefore...
每次遇到IDENTIFIER令牌时，我们都必须查找范围链。 Javascript VM包含每个执行上下文的激活对象，其中可能包含arguments对象，本地定义的变量等。如果我们无法在Activation对象中找到它，我们将开始查找范围链，直到我们到达全局范围。如果没有找到，我们抛出一个 ReferenceError 。
We have to look up the scope chain each time we encounter an IDENTIFIER token. Javascript VMs contain an "Activation object" for each execution context which may contain the "arguments" object, locally defined variables, etc. If we cannot find it in the Activation object, we begin looking up the scope chain until we reach the global scope. If nothing is found, we throw a ReferenceError.
一旦我们找到了变量声明，我们调用构造函数。 new Array 是一个隐式函数调用，经验法则是函数调用在执行期间较慢（因此静态C / C ++编译器允许函数内联 - 这是哪个JS像SpiderMonkey这样的JIT引擎必须在运行中执行）
Once we've located the variable declaration, we invoke the constructor. new Array is an implicit function call, and the rule of thumb is that function calls are slower during execution (hence why static C/C++ compilers allow "function inlining" - which JS JIT engines such as SpiderMonkey have to do on-the-fly)
数组构造函数被重载。 Array构造函数实现为本机代码，因此它提供了一些性能增强，但仍需要检查参数长度并相应地执行操作。此外，如果只提供了一个参数，我们需要进一步检查参数的类型。 new Array（foo）产生[foo]，其中新的Array（1）产生[undefined]
The Array constructor is overloaded. The Array constructor is implemented as native code so it provides some performance enhancements, but it still needs to check for arguments length and act accordingly. Moreover, in the event only one argument is supplied, we need to further check the type of the argument. new Array("foo") produces ["foo"] where as new Array(1) produces [undefined]
所以为了简化它：使用数组文字，VM知道我们想要一个数组;使用 new Array ，VM需要使用额外的CPU周期来确定新数组 实际确实。
So to simplify it all: with array literals, the VM knows we want an array; with new Array, the VM needs to use extra CPU cycles to figure out what new Array actually does.
这篇关于为什么arr = []比arr = new Array更快？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！
