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In most cases, expressions are interested in the apparent type of the
contextual type. For instance:
var x = { hasOwnProperty(prop) { /* ... */ };
In the above, 'prop' should be contextually typed as 'string' from the
signature of 'hasOwnProperty' in the global 'Object' type.
However, in the case of string literal types, we don't want to get the
apparent type after fetching the contextual type. This is because the
apparent type of the '"onload"' string literal type is the global 'String'
type. This has adverse effects in simple assignments like the following:
let x: "onload" = "onload";
In this example, the right-hand side of the assignment will grab the type
of 'x'. After figuring out the type is "onload", we then get the apparent
type which is 'String'. This is problematic because when we then check the
assignment itself, 'String's are not assignable to '"onload"'s.
So in this case, we grab the contextual type *without* getting its
apparent type.
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 Jake tools and dev dependencies:
npm install -g jake
npm install
Use one of the following to build and test:
jake local # Build the compiler into built/local
jake clean # Delete the built compiler
jake LKG # Replace the last known good with the built one.
# Bootstrapping step to be executed when the built compiler reaches a stable state.
jake tests # Build the test infrastructure using the built compiler.
jake 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>.
jake runtests-browser # Runs the tests using the built run.js file. Syntax is jake runtests. Optional
parameters 'host=', 'tests=[regex], reporter=[list|spec|json|<more>]'.
jake baseline-accept # This replaces the baseline test results with the results obtained from jake runtests.
jake lint # Runs tslint on the TypeScript source.
jake -T # 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.