Add `--inlineConsts` flag

New flag --inlineConsts

Input:

const x = 'hello';

console.log(x);

Output:

console.log('hello');

See also #6678 + #6804

TBD: It’s not clear you would want longer strings to inline into their use sites. Discuss.

[alitaheri]

This feature can actually decrease file size sometimes when combined with tools like uglify. Usually with conditionals:

const myConst = "blah";

// ...

// this code can be safely removed after myConst gets replaced with "blah"
if (myConst !== "blah") {
 // ...
}

[dead-claudia]

@Raiondesu declare in TS is about declaring bindings, specifically bindings with no immediate value, not changing their instantiation.

In my experience, I’ve never once stumbled across a use case where a primitive constant would’ve been better not inlined, even in perf-critical code. If you’re worried about duplication, gzip can make quick work of that. About the only two places I could imagine it being useful to not inline a constant is in embedded applications and with very large strings used in multiple unrelated places. In the first, you need enough control that you’d do better (ab)using const enums for constants than using const + --inlineConsts. In the second, you can work around it with wrapping it in an object, but it doesn’t appear large strings are guaranteed to make it, either.