JavaScript字符串连接速度 [英] JavaScript string concatenation speed

问题描述

有人可以向我解释这个：

Can someone explain this one to me:

http://jsperf.com/string-concatenation-1/2

如果你很懒，我测试了A）vs B） ：

If you're lazy, I tested A) vs B):

A）

var innerHTML = "";

items.forEach(function(item) {
    innerHTML += item;
});

B）

var innerHTML = items.join("");

两个测试的项目是相同的500个元素的字符串数组，每个字符串是随机的，长度在100到400个字符之间。

Where items for both tests is the same 500-element array of strings, with each string being random and between 100 and 400 characters in length.

A）最终快10倍。怎么会这样 - 我一直认为使用 join（）连接是一个优化技巧。我的测试有什么问题吗？

A) ends up being 10x faster. How can this be--I always thought concatenating using join("") was an optimization trick. Is there something flawed with my tests?

推荐答案

使用加入（）是在IE6上组合大字符串的优化技巧，以避免 O（n ** 2）缓冲区副本。由于 O（n ** 2）只能真正控制数组的开销，因此从来没有预料到组成小字符串的巨大性能胜利。

Using join("") was an optimization trick for composing large strings on IE6 to avoid O(n**2) buffer copies. It was never expected to be a huge performance win for composing small strings since the O(n**2) only really dominates the overhead of an array for largish n.

现代翻译通过使用依赖字符串来解决这个问题。有关依赖字符串的说明以及一些优缺点，请参阅此 mozilla bug 。

Modern interpreters get around this by using "dependent strings". See this mozilla bug for an explanation of dependent strings and some of the advantages and drawbacks.

基本上，现代口译员知道许多不同类型的字符串：

Basically, modern interpreters knows about a number of different kinds of strings:

1. 字符数组


2. 另一个字符串的切片（子字符串）


3. 两个其他字符串的串联

这使得连接和子串O（1）的代价有时会使一个子网状缓冲区过多，从而导致垃圾收集器效率低下或复杂化。

This makes concatenation and substring O(1) at the cost of sometimes keeping too much of a substringed buffer alive resulting in inefficiency or complexity in the garbage collector.

一些现代解释器已经开始考虑进一步将（1）分解为仅用于ASCII字符串的byte [] s，以及当字符串包含UTF-16代码时uint16s的数组不能装入一个字节的单位。但我不知道这个想法是否真的在任何翻译中。

Some modern interpreters have played around with the idea of further decomposing (1) into byte[]s for ASCII only strings, and arrays of uint16s when a string contains a UTF-16 code unit that can't fit into one byte. But I don't know if that idea is actually in any interpreter.

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