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
* Fix#10083 - allowSyntheticDefaultImports alters getExternalModuleMember (#10096)
* Add a helper function `getOrUpdateProperty` to prevent unprotected access to Maps.
* Limit type guards as assertions to incomplete types in loops
* Accept new baselines
* Fix linting error
* [Release-2.0] Fix 9662: Visual Studio 2015 with TS2.0 gives incorrect @types path resolution errors (#9867)
* Change the shape of the shim layer to support getAutomaticTypeDirectives
* Change the key for looking up automatic type-directives
* Update baselines from change look-up name of type-directives
* Add @currentDirectory into the test
* Update baselines
* Fix linting error
* Address PR: fix spelling mistake
* Instead of return path of the type directive names just return type directive names
* Remove unused reference files: these tests produce erros so they will not produce these files (#9233)
* Don't allow properties inherited from Object to be automatically included in TSX attributes
* Port PR #10016 to Master (#10100)
* Treat namespaceExportDeclaration as declaration
* Update baselines
* wip - add tests
* Add tests
* Show "export namespace" for quick-info
* Update baselines from merging
Kanchalai Tanglertsampan
Nathan Shively-Sanders
Anders Hejlsberg
Andy Hanson
Ryan Cavanaugh
Yui
Wesley Wigham
Richard Knoll
Godfrey Chan
Paul van Brenk