The TypeScript team is investigating Async/Await. What follows is a proposal for how Async/Await should be implemented. It embraces the widely accepted community standard for syntactically sane asynchronous programming: Promises/A+ spec. By using promises as the atomic unit for async/await in TypeScript, the language taps into a large, existing set of libraries (Q, WinJS, DOM Futures proposal, etc.), making them instantly compatible.
The need is clear - asynchronous behavior, and the handling of that behavior, can be difficult to reason about in
JavaScript. TypeScript does little currently to address this problem. The Promises/A+ spec gets us most of the way
there, making it possible to develop with asynchronous requests in a way that seems synchronous. There are still
certain things the spec alone can not handle well - using promises directly in conditional statements, awaiting in
loops, and assigning to a typed value using both an async function call and synchronously, depending on a branching
statement.
- Be able to handle all use cases covered by async/await in C#.
- Output that can be easily grokked by un-initiated. This means having absolutely no mystery in the source transformations.
- Ability to convert other asynchronous operations into promises.
- Start with a simple syntax definition. I.e. this is how the code will look like when you write it.
- Go through each situation where the await keyword introduces complexity in implementation from most basic to most advanced.
- Each section will break down the syntax until it fits a pattern shown in a previous section or is valid current TypeScript.
See AsyncAwesome.cs. Notice there are a few different use cases that must be covered to consider our implementation a success:
awaitassignment can exist within a condition. In fact, the function can never await a task due to the branches taken, but still return the syncrhonously-assigned value wrapped in a task.- Tasks can be
awaited in a loop synchronously. The next step will not be completed until the previous iteration completes. awaits can be performed inside of conditions or as function parameters
- Variable Hoisting
- Binary Operators
- Short-Circuiting Boolean Operators
- Conditional Expressions
- Loops
- Switch
- Try-catch-finally
- Expressions
- Implicit Promise Interface
- Async Function Expression
- Defer Function Expression
- Await Expression
- Code Generation
- Async Function
- Defer Function
- Simple Await Expression
- Await Expressions in Compound Expressions
- Await in Boolean Expressions
- Await in Conditionals
- Await in Switch
- Await in Loops