repl: use String#repeat instead of Array#join

[evanlucas]

String#repeat is quite a bit faster than new Array().join().

[Fishrock123]

LGTM, evidence of said perf would be nice of course.

[evanlucas]

In the common case here:

This is testing:

' '.repeat(1)
// vs
new Array(2).join(' ')

String#repeat 14.5 ns/op
Array#join 255.3 ns/op

[cjihrig]

LGTM. The speedup is nice. Not sure how important it is in the REPL.

[evanlucas]

The advantage is the speedup in showing the prompt IMO

[cjihrig]

I don't think anyone will perceive < 1us :-)

It's still nice to know for more performance critical places.

[evanlucas]

yea, true

[ronkorving]

It seems that the only place where this pattern emerged is in repl.js, unless my grep is not revealing enough.

$ grep -r 'new Array(' .
./dns.js: var args = new Array(arguments.length + 1);
./domain.js: var args = new Array(len - 1);
./events.js: args = new Array(len - 1);
./events.js: var copy = new Array(i);
./fs.js: const chunks = new Array(len);
./internal/child_process.js: // Don't concat() a new Array() because it would be sparse, and
./net.js: var chunks = new Array(data.length << 1);
./querystring.js:var hexTable = new Array(256);
./repl.js: var levelInd = new Array(this.lines.level.length).join('..');
./repl.js: self.lines.push(new Array(self.lines.level.length).join(' ') + cmd);
./timers.js: var args = new Array(length - 2);
./timers.js: var args = new Array(length - 2);
./timers.js: args = new Array(len - 1);
./util.js: var output = new Array(value.length);

[silverwind]

This could fit on a single line.

[silverwind]

LGTM with style nit. By the way: https://jsperf.com/string-repeat-native-vs-array-join-vs-loop

[evanlucas]

Fixed the nit

CI: https://ci.nodejs.org/job/node-test-pull-request/1038/

[silverwind]

Please this one too :)

[evanlucas]

ahh missed that one. Fixed

[silverwind]

CI is happy, let's land it!

[evanlucas]

k, landing now

[evanlucas]

Landed in 50125e2. Thanks!