This document outlines our focused tasks for TypeScript 3.5, as well as some of the discussion that explains how/why we prioritized certain work items. Nothing is set in stone, but we will strive to complete them in a reasonable timeframe.

Dates

Here's the release plan.

1. May 10th: Create a release-3.5 branch, and update the version to 3.5.0-rc. master is locked from receiving new feature-work that is not 3.5-bound.
   1. Start a build of TypeScript for Visual Studio for our remote testing team.
2. May 15th: Last day to adjust the release-3.5 branch with any feedback we receive.
   1. Kick off a final Visual Studio build if necessary.
   2. (2pm Pacific) Produce an insiders release for VS Code.
3. Morning of May 16th: The release candidate goes out.
4. May 24th: Merge master into release-3.5 one last time, finalize the version as 3.5.1. master is unlocked. Critical changes for 3.5 must be ported in.
   1. Start a build of TypeScript for Visual Studio for our remote testing team.
5. May 29th: Last day to adjust the release-3.5 branch with any feedback we receive.
   1. Kick off a final Visual Studio build if necessary.
   2. (2pm Pacific) Produce an insiders release for VS Code which is identical to the final release.
6. May 30th: Announcing TypeScript 3.5 🚀

Work Items

Expected Work Items

• Language
  • Investigate negated types 🏃🏻‍
  • globalThis expressivity fixes
  • Strongly typed iterators and generators
• Compiler
  • Anticipated --incremental fixes and improvements
  • --declaration and --allowJs
  • --declaration and --isolatedModules
  • APIs for --incremental and composite projects (--build mode) 🏃
  • Investigate TypeScript plugin APIs 🏃
• Language Service
  • Smart Select API
  • Sorted/prioritized completions
  • Extract type
• Infrastructure
  • Automated testing infrastructure for language service on real world code 🏃🏻‍
  • Migrate to ESLint 🏃🏻‍

(🏃🏻‍ means work has already started)

Deferred Work Items

• Arbitrary index signatures
• Call hierarchy

Planning Meeting Notes

Motivations

• Goals and current 6-month roadmap
• GitHub user feedback (👍s)
• Feedback from customer interviews and social media.
• Visual Studio and Visual Studio Code feedback and new functionality demands
• Actionable PRs (need to make a call)
• Bug backlog

General (compiler/infrastructure/reliability)

• Negated types

  • Rationale: Core type system
  • Uses: control flow analysis on generic/"higher order" types
  • Problems: But you end up with lots of vacuous intersections that are "garbage" types.
  • Will likely lead to a tail of bugs, at least initially
  • Action item: need to prototype, see what sort of breakage we see with implementations of integration with control-flow analysis, and see what the perf impact is.
  • ✅ Work on this.

• Arbitrary index signatures

  • Rationale: Core type system
  • Not a must for negated types
  • But like to get them done first.
  • Also need a unifying concept between them and mapped object types.
  • Resolution: Defer

• Compiler API prototyping

  • Rationale: minification, 3rd party tools integration
  • In progress
  • ✅

• Expecting --incremental build issues

  • Rationale: Stability, scaling, performance
  • ✅

• Crawler work for testing TSServer

  • Rationale: Stability
  • Context:
    • IntelliCode team crawls public code to train on, but leverages TSServer.
    • Turns out that it was a great way to test TSServer and catch bugs!
    • We want to leverage it to get a good litmus that it's stable.
    • Give partner teams assurance that they can adopt nightly TypeScripts.
  • ✅

• TSLint -> ESLint migration

  • Internal engineering
  • ✅

• New engineer starting!

  • Ramping-up bugs
  • Website
    • Education and UX
  • ✅

• globalThis-related work

  • Bug and standards involvement
  • Issues with block-scoped declarations interfacing with the global object.
  • ✅

• --declaration with --allowJs

  • TypeScript beyond TypeScript
  • ✅ 🎉

• --declaration with --isolatedModules

  • User demand
  • Aside: Ensure this all works with composite.
  • Resolution: investigate

Productivity

• Smart Select API
  • TSServer: Smart select API #29071
  • VS Code ask, but generally applicable editor integration
  • ✅
• Sorted/prioritized completions
  • favor local types and values in autocomplete popup over ambients declaration #15024
  • Editor quality improvements
  • This and smart select have been around for long enough that they get priority.
  • ✅
• Extract type refactoring
  • Suggestion: Refactor extract to type alias/typedef #23869
  • Shouldn't it just always work?
    • <T>(x: T): { value: T }
    • It has all the same problems as extract method!
      • Well you don't have to worry about control flow.
  • Could just do this for non-generic types for the minimal-viable product (MVP).
  • Resolution: try to get done for 3.5. ✅
• Call hierarchy breadcrumbs
  • Feature request: Call Hierarchy vscode#16110
  • VS Code ask, but generally applicable editor integration
  • Resolution: Probably will defer to next version.