+ =，++，+之间的性能差异

Question

我创建了这个测试http://jsperf.com/loop-counter为什么这三个表达式之间存在这样的差异。

Answer 1

because your test is wrong. you're reusing the same variable, so the larger it gets, the slower it is to increment. take a look at this: http://jsperf.com/loop-counter/6

this is how jsperf works - preparation code is run only once, before all tests.

Answer 2

I tried running all three tests several times, and each time I reload the page, the first test I try is the fastest by far.

So I'm guessing there is some issue with the test being too short, ie the code that runs the tests is taking up most of the time.

Answer 3

如果这不是一个修辞问题而你实际上想要一个aswer：那么人们如何在浏览器中编写JS引擎。

Answer 4

It's because of what the program is doing behind the scenes:

l_count += 1; This adds the number 1 to the variable.

l_count = l_count + 1; This calls the variable l_count, reads it, adds 1 to the result, and passes that back to l_count.

l_count++; This adds 1 to the variable after the line is run. So the value is stored in another temporary variable while the line is done, then the value is returned, added 1 and saved back to the original value.