省略分号会影响JavaScript的性能吗？

Question

I was discussing with a colleague about JavaScript while looking at some snippets. We noticed that these snippets were missing the ; at the end of the statements. We all know that JS is interpreted correctly even if no semicolon is shown at the end of a line, but I was wondering if this affects somehow the performance of evaluation, since it is an interpreted language.

Answer 1

A javascript file with spaces, semi-colons and comments is heavier. That's the main impact.

But you're a coder and you have to maintain the code, so this very slight impact is much less important than the adverse one on readability. And omitting the semicolons means you know when you can omit them. But the rules aren't so simple and learning them isn't worth your time.

Leave the semi-colons where they are, you'll avoid bugs.

And use a minifier to build a more concise code for the browser if you want to have the lightest possible code. It's its duty, not yours.

Answer 2

Omitting semicolons in JS is big debate, but we should always keep semicolon. If you talk about performance, then there will be very little benefit while keeping semicolon.

But the things not end up here only. Other than performance there is one big thing which needs to be take care.

Doug Crockford explains the need of semicolons very well in this presentation :

The JS Interpreter finds an error, adds a semicolon, and runs the whole thing again. But not every time he puts the semicolon on the right place and hilarious bugs are the consequence. You should always make semicolons and run your js through testing tools like JSLint.

Other than this semicolons give the code more structure and make it cleaner - additionally, their presence allows some developers to obfuscate their code.

Hope it will help you.

Answer 3

Referring to the test provided: That really doesn't illustrate the performance difference of ASI vs non ASI. The performance that will be impacted is likely in tokenization. The interpreter or JIT won't be doing this for each iteration, but only once. The effect would likely only be noticed when loading a large code base. The semicolon allows the JIT or interpreter to more quickly recognize an end-of-statement than inferring it from the next tokens.

Answer 4

Tiny differences in code syntax usually have very little effect on the performance of the code. the process of interpreting a string of code is very efficient and takes a tiny fraction of the time taken by actually running the code.

An inefficient algorithm or an extra network call, such as pulling in multiple .js files instead of a single one have far greater impact.