Add a "specify type" command to tsserver

Suggestion

🔍 Search Terms

List of keywords you searched for before creating this issue. Write them down here so that others can find this suggestion more easily and help provide feedback.

✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

I would like to be able to highlight an expression/binding and have typescript let me input the type it should be instead of inferring the type and/or replacing the current type.

📃 Motivating Example

// a .js file, as I use JSDoc mostly
function double(a) {
  return a * 2;
}

I would like to be able to highlight a and have a JSDoc in JS or type position in TS open up for me to input the type number manually. In particular figuring out nesting for "casting" in JS can be confusing if applied to non-bindings:

let a = JSON.parse(_);

💻 Use Cases

I mostly want to use this for tooling that can integrate against existing code without my cursor jumping around to manually specify types. Having this be done and placing my cursor back where it was would be mildly convenient. Having it as an API would also let me do this automatically with some tooling that works on existing code instead of just code generators.

The request targets tsserver and describes a command that operates on a highlighted expression or binding. Start by tracing tsserver's existing commands for source edits and type information; done would mean accepting a manually entered type in JavaScript JSDoc or TypeScript type position, exposing it for tooling, and preserving the cursor location.