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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
TypeScript
TypeScript is a language for application-scale JavaScript. TypeScript adds optional types, classes, and modules to JavaScript. TypeScript supports tools for large-scale JavaScript applications for any browser, for any host, on any OS. TypeScript compiles to readable, standards-based JavaScript. Try it out at the playground, and stay up to date via our blog and Twitter account.
Installing
For the latest stable version:
npm install -g typescript
For our nightly builds:
npm install -g typescript@next
Contribute
There are many ways to contribute to TypeScript.
- Submit bugs and help us verify fixes as they are checked in.
- Review the source code changes.
- Engage with other TypeScript users and developers on StackOverflow.
- Join the #typescript discussion on Twitter.
- Contribute bug fixes.
- Read the language specification (docx, pdf, md).
Documentation
Building
In order to build the TypeScript compiler, ensure that you have Git and Node.js installed.
Clone a copy of the repo:
git clone https://github.com/Microsoft/TypeScript.git
Change to the TypeScript directory:
cd TypeScript
Install Gulp tools and dev dependencies:
npm install -g gulp
npm install
Use one of the following to build and test:
gulp local # Build the compiler into built/local
gulp clean # Delete the built compiler
gulp LKG # Replace the last known good with the built one.
# Bootstrapping step to be executed when the built compiler reaches a stable state.
gulp tests # Build the test infrastructure using the built compiler.
gulp runtests # Run tests using the built compiler and test infrastructure.
# You can override the host or specify a test for this command.
# Use host=<hostName> or tests=<testPath>.
gulp runtests-browser # Runs the tests using the built run.js file. Syntax is gulp runtests. Optional
parameters 'host=', 'tests=[regex], reporter=[list|spec|json|<more>]'.
gulp baseline-accept # This replaces the baseline test results with the results obtained from gulp runtests.
gulp lint # Runs tslint on the TypeScript source.
gulp help # List the above commands.
Usage
node built/local/tsc.js hello.ts
Roadmap
For details on our planned features and future direction please refer to our roadmap.