Template strings transpile incorrectly

[ljharb]

The following:

`${foo}`

transpiles to:

"" + foo;

However, concatenating to an empty string actually calls valueOf, not toString, and as such is not correct. See the following JS:

var x = { toString: function () { return NaN; }, valueOf: function () { return Infinity; }};
[x.toString(), x + '', String(x)]
/* note that only the third item is both a string, and the toString value */

It should transpile to:

String(foo)

[sebmck]

Duplicate of #1065 and also see the optional es6.spec.templateLiterals transformer.

[ljharb]

Ah, thanks, i didn't see the other issue. I'm glad the optional transformer is there, but imo that should be default, since the default case is not going to be using template strings in a tight loop - I'd rather see incorrectness+performance be opt-in than the way it is right now.

[ljharb]

In other words, correctness is more important than performance, and for the few who care, they'll be able to figure out how to turn on any relevant options to increase performance at the cost of known spec violations.