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cc2dc3acb0
When there are property assignments in a the class body of an inheriting
class, tsc current emit the following compilation:
```ts
class Foo extends Bar {
public foo = 1;
}
```
```js
class Foo extends Bar {
constructor(…args) {
super(…args);
this.foo = 1;
}
}
```
This introduces an unneeded local variable and might force a reification
of the `arguments` object (or otherwise reify the arguments into an
array).
This is particularly bad when that output is fed into another transpiler
like Babel. In Babel, you get something like this today:
```js
var Foo = (function (_Bar) {
_inherits(Foo, _Bar);
function Foo() {
_classCallCheck(this, Foo);
for (var _len = arguments.length, args = Array(_len), _key = 0; _key < _len; _key++) {
args[_key] = arguments[_key];
}
_Bar.call.apply(_Bar, [this].concat(args));
this.foo = 1;
}
return Foo;
})(Bar);
```
This causes a lot of needless work/allocations and some very strange
code (`.call.apply` o_0).
Admittedly, this is not strictly tsc’s problem; it could have done a
deeper analysis of the code and optimized out the extra dance. However,
tsc could also have emitted this simpler, more concise and semantically
equivalent code in the first place:
```js
class Foo extends Bar {
constructor() {
super(…arguments);
this.foo = 1;
}
}
```
Which compiles into the following in Babel:
```js
var Foo = (function (_Bar) {
_inherits(Foo, _Bar);
function Foo() {
_classCallCheck(this, Foo);
_Bar.apply(this, arguments);
this.foo = 1;
}
return Foo;
})(Bar);
```
Which is well-optimized (today) in most engines and much less confusing
to read.
As far as I can tell, the proposed compilation has exactly the same
semantics as before.
Fixes #10175