Rule proposal: Prefer String#startsWith() and String#endsWith()

[sindresorhus]

The rule would catch the following cases:

.startsWith instead of:

• foo.indexOf('bar') === 0
• foo[0] === 'b'
• foo.charAt(0) === 'b' (and foo.charAt())
• foo.slice(0, 2) === 'bar' (and .substring & .substr)
• /^bar/.test(foo) (and .match())

.endsWith instead of:

• .lastIndexOf
• foo[foo.length - 1] === 'r'
• foo.charAt(foo.length - 1) === 'b'
• foo.slice(0, -3) === 'bar' (and .substring & .substr)
• /bar$/.test(foo) (and .match())

The regex case is especially useful as I've seen a lot of regexes used in this scenario.

[mysticatea]

I will work on this after accepted by the core team.

[bradzacher]

I don't necessarily agree that
foo[0] === 'b' is equivalient to foo.startsWith('b') (and same for the endsWith example)
Purely because using startsWith for a single character will be significantly slower than using the the character indexer:
https://www.measurethat.net/Benchmarks/Show/3319/0/startswith-vs-charat-for-single-character

[sindresorhus]

@bradzacher I'm not convinced by a micro-benchmark. Whether this actually matters in production code is unclear. Also, startsWith is a fairly recent addition and has not had time to be properly optimized by V8 yet. Most devs use other newer JS features, like for-of instead of for even though for is still faster (in micro-benchmarks).

[bradzacher]

I went and found that benchmark to confirm my thinking was correct.

charAt/index access should dive straight into the memory via pointer arithmetic - i.e. should be a near free, super cheap operation.
startsWith will require it to set up a new stack frame, initialise whatever internal variables, and start a loop (which iterates once).

The overhead of that clearly adds up!

You're right though that they might consider adding a special code path for starts/endsWith being called with a single character.

We would probably want to add some settings to allow people to customise what's checked.

[sindresorhus]

You're right though that they might consider adding a special code path for starts/endsWith being called with a single character.

https://twitter.com/ryzokuken/status/1096281036403691520 :)

We would probably want to add some settings to allow people to customise what's checked.

Sure. I think foo[0]/foo.charAt(0) is really the only case that needs a setting.

[bradzacher]

Haha! I didn't know you had a direct line to the V8 devs :P
That's sick though. If there looking to optimise it asap, then in that case I wouldn't even worry about a setting. We can just document it in the readme to head off any possible enhancement requests :)