Unable to extend classes which constructors require new when targeting es5

[falsandtru]

TypeScript Version: master

Code

new class extends Map {}();

Expected behavior:

$ node built/local/tsc.js -t es5 --lib es6 index.ts && node index.js

Actual behavior:

$ node built/local/tsc.js -t es5 --lib es6 index.ts && node index.js
...\index.js:9
        return _super.apply(this, arguments) || this;
                      ^

TypeError: Constructor Map requires 'new'

[DanielRosenwasser]

I'll note that this doesn't work in 2.0 or in Babel either.

[falsandtru]

Hm, seems like there is no way to fix this bug. I think, so new.target is added in es6.

[DanielRosenwasser]

I think super calls could potentially be emitted as

_this = new (Function.prototype.bind.apply(_super, [null].concat(arguments)))

but I think that would have a pretty bad performance impact.

[falsandtru]

Then can you make _this as an instance of both base and derived classes for instanceof operator?

[ghost]

See also #10853 which is closed, even though the issue is not resolved.

[RyanCavanaugh]

This doesn't seem like an important scenario for modern JS development - folks can either target ES6, or use an alternate transpiler.