Bug 188882 – String.concat() significantly slower than '+' operator

• Status: NEW
• Component: JavaScriptCore

[Nicholas Shanks]

When String.prototype.concat() and String.prototype.join() have not been modified from their initial values, promotion of their arguments to strings and concatenation of those strings should be nearly as fast as the '+' operator with one string operand (which also has to promote the other operand). This will allow functional composition to be performant.

See https://jsperf.com/concat-vs-plus-vs-join-2

[Radar WebKit Bug Importer]

<rdar://problem/43646782>

[Keith Miller]

It looks like you are seeing such a large difference because the test is passing more than one argument to concat at the same time. This causes us to emit a loop which is slowing that code down.

If you change your test to:

string1.concat(string2).concat(string3);

you will see that the perf is quite close. In theory we could do some loop unrolling to fix this but the same issue applies to calling the '+' operator in a loop.

The join case is slow because it will allocate an Array, which is expensive. We could also optimize this but it's probably easier to use the concat method for now.