94f3ca142f
As per the linked issue, although TypeScript already has checking in place that tries to prevent users from extending classes before their declaration, it's still possible to accidentally work around the checker. This adds a block to `__extends` that throws a `TypeError` if the base class `b` isn't a function.
My _hope_ is that this will not have a negative performance impact on community performance-critical applications, as they would likely already prefer newer browser/Node versions and output ES2015+ code. If there is something you can think of that I should do to verify that hope, I'd love to know!
For reference, runtime errors in Node 12.0.0 (Chrome exhibits the same messages):
```js
class X extends null { }
// undefined
class Y extends undefined { }
// TypeError: Class extends value undefined is not a constructor or null
class Z extends 0 { }
// TypeError: Class extends value 0 is not a constructor or null
```
`Class extends value {0} is not a constructor or null` matches the Node.js behavior:
* [Message template](2bdeb88c27/deps/v8/src/common/message-template.h) for `ExtendsValueNotConstructor`
* [Error thrown with that message](6ca81ad72a/deps/v8/src/runtime/runtime-classes.cc (L617)) when `!super_class->IsConstructor()`
Runtime errors in Firefox 72.0.1:
```js
class X extends null { }
// undefined
class Y extends undefined { }
// TypeError: class heritage undefined is not an object or null
class Z extends 0 { }
// TypeError: class heritage 0 is not an object or null
```
TypeScript
TypeScript is a language for application-scale JavaScript. TypeScript adds optional types to JavaScript that support 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.
Find others who are using TypeScript at our community page.
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.
- Help each other in the TypeScript Community Discord.
- Join the #typescript discussion on Twitter.
- Contribute bug fixes.
- Read the language specification (docx, pdf, md).
This project has adopted the Microsoft Open Source Code of Conduct. For more information see the Code of Conduct FAQ or contact opencode@microsoft.com with any additional questions or comments.
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.
# Some low-value tests are skipped when not on a CI machine - you can use the
# --skipPercent=0 command to override this behavior and run all tests locally.
# You can override the specific suite runner used or specify a test for this command.
# Use --tests=<testPath> for a specific test and/or --runner=<runnerName> for a specific suite.
# Valid runners include conformance, compiler, fourslash, project, user, and docker
# The user and docker runners are extended test suite runners - the user runner
# works on disk in the tests/cases/user directory, while the docker runner works in containers.
# You'll need to have the docker executable in your system path for the docker runner to work.
gulp runtests-parallel # Like runtests, but split across multiple threads. Uses a number of threads equal to the system
# core count by default. Use --workers=<number> to adjust this.
gulp baseline-accept # This replaces the baseline test results with the results obtained from gulp runtests.
gulp lint # Runs eslint 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.