https://devblogs.microsoft.com/typescript/announcing-typescript-4-9-rc/

This issue is just to track all of the new features and their implementation state in this project.
As with all releases, we will not necessarily to support all features until closer to the full release when everything the features are stabilised.

Please be patient.

✅ The satisfies Operator (#5717)

constpalette={red:[255,0,0],green:"#00ff00",bleu:[0,0,255]//  ~~~~ The typo is now caught!}satisfiesRecord<Colors,string|RGB>;

This will require AST changes

✅ Auto-Accessors in Classes (#5926)

classPerson{    accessorname:string;constructor(name:string){this.name=name;}}

This will require AST changes.
This feature is based on the stage-3 decorators proposal.
We will need to implement the definedESTree AST

✅substitute Replaced Withconstraint onSubstitutionTypes

We will need to double-check our codebase for usages of the old property.

✅lib.d.ts Updates

We will need to regenerate our types withinscope-manager.

Other changes with no impact to us

• Unlisted Property Narrowing with thein Operator
• Checks For Equality onNaN
• File-Watching Now Uses File System Events
• "Remove Unused Imports" and "Sort Imports" Commands for Editors
• Go-to-Definition onreturn Keywords
• Performance Improvements
• Breaking Changes
  • Better Types forPromise.resolve
  • JavaScript Emit No Longer Elides Imports
  • exports is Prioritized OvertypesVersions