Upgrade to TypeScript 6.0

[mcmire]

TypeScript 7 will ship with the Go version of the TypeScript compiler, making it much faster for us to run the lint step.

TypeScript 6 is a compatibility version that will make it relatively seamless to upgrade to TypeScript 7.

[mcmire]

I ran an analysis through AI and got this:

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TypeScript 6.0 Beta Migration Analysis for metamask/core

Current State

• TypeScript version: ~5.3.3 (uniform across all 74 packages + root)
• Key tsconfig settings (in tsconfig.base.json): strict: true, module: "Node16", moduleResolution: "Node16", target: "ES2020", composite: true, esModuleInterop: true, isolatedModules: true

1. Configuration Changes Required

Good news first: Since this repo already explicitly sets strict, module, moduleResolution, target, and esModuleInterop in tsconfig.base.json, many of TS 6.0's changed defaults are irrelevant -- they only affect projects relying on implicit defaults.

1.1 types default changed to [] (HIGH IMPACT)

TS 6.0 no longer auto-discovers @types/* packages. No tsconfig in the repo explicitly sets types, so @types/node, @types/jest, etc. will stop being included automatically.

Action: Add "types": ["node"] to tsconfig.base.json. For test configs (or the Jest setup), ensure @types/jest is also included. The per-package tsconfig.json files used for editor/lint (which include test files) may need "types": ["node", "jest"].

1.2 baseUrl is deprecated (MEDIUM-HIGH IMPACT)

Every single package's tsconfig.json and tsconfig.build.json sets baseUrl: "./". In TS 6.0, baseUrl is deprecated (error, suppressible with "ignoreDeprecations": "6.0", removed entirely in TS 7.0).

Action: Remove baseUrl from all ~150 tsconfig files. Since baseUrl is "./" everywhere and paths is only defined in tsconfig.packages.json, the paths entries need to be updated to include the baseUrl prefix. Currently paths like "@metamask/*": ["../*/src"] resolve relative to each package's baseUrl (i.e., ./<package-dir>/). Without baseUrl, paths entries resolve relative to the tsconfig file they're defined in. Since the paths are defined in tsconfig.packages.json (at the repo root) and inherited by each package, this may need restructuring. The ts5to6 migration tool can automate this.

1.3 moduleResolution: "Node16" -- NO change needed

The deprecation is for "node" / "node10" / "classic", not for "Node16" / "NodeNext". The repo uses "Node16", which remains valid.

1.4 esModuleInterop: false is deprecated -- NO change needed

The repo already sets esModuleInterop: true.

1.5 noUncheckedSideEffectImports defaults to true (LOW IMPACT)

Side-effect-only imports (import "./something") will now be checked for resolution. Need to verify the codebase doesn't have unresolvable side-effect imports.

1.6 rootDir default changed to . -- NO change needed for builds

Every tsconfig.build.json already explicitly sets rootDir: "./src". However, ~61 package tsconfig.json (dev/editor) files do NOT set rootDir and previously relied on inference. With noEmit: true (set in the root tsconfig.json), this shouldn't affect builds, but may generate warnings.

2. Package Upgrades Required

Package	Current	Needed	Notes
typescript	~5.3.3	~6.0.0-beta	Core upgrade across root + 74 packages
typescript-eslint	^8.48.0	^8.57+ (once TS6 PR merges)	PR #12124 is in progress
@typescript-eslint/eslint-plugin	^8.48.0	Same as above
@typescript-eslint/parser	^8.48.0	Same as above
ts-jest	^29.2.5	TBD (no TS6 support yet)	Potential blocker
typedoc	^0.25.13	^0.28.x + TS6 patch	Potential blocker -- 3 major versions behind, and even latest 0.28.x doesn't support TS6 yet
@ts-bridge/cli	^0.6.4	TBD (needs testing)	Low-to-medium risk; uses TS compiler API internally
eslint-import-resolver-typescript	^3.6.3	May need update	Depends on how it handles baseUrl removal

3. Incompatible / Blocking Tools

ts-jest -- HIGH RISK BLOCKER

• No TS 6.0 support work has started. No issue or PR exists.
• Deep integration with the TypeScript compiler API (ts.transpileModule).
• Historically slow to support new TS majors (TS 5.6 support took months).
• Mitigation: Consider switching to @swc/jest (Rust-based, type-stripping transpiler that doesn't depend on tsc) or using babel-jest with @babel/preset-typescript. Both are significantly faster and don't depend on the TS compiler API.

TypeDoc -- HIGH RISK BLOCKER

• Current version (0.25.13) has a peerDependency on TypeScript 4.6-5.x.
• Latest version (0.28.17) also only supports TS 5.x.
• Issue #3084 is open but unresolved.
• Mitigation: TypeDoc is only used for doc generation (build:docs), not for the core build/test pipeline. You could defer upgrading TypeDoc and keep TS 5.x installed alongside TS 6.x just for doc generation, or temporarily skip doc generation.

@ts-bridge/cli -- MEDIUM RISK

• No confirmed TS 6.0 support, but claims compatibility with "TS 4.8 or later."
• Since it's maintained by a MetaMask team member (@Mrtenz), fixes can likely be made quickly if needed.

4. Enum Usage (138 enums across 71 files)

TypeScript 6.0 does NOT remove enums or enable erasableSyntaxOnly by default. The 138 enums in the codebase will continue to work fine. However, this is the direction TypeScript is heading (TS 7.0 / Node.js native TS execution only works with erasable syntax), so it's worth flagging for future planning.

No parameter properties or legacy module Foo {} namespace syntax is used, which is good.

5. Things That Are Fine (No Action Needed)

• strict: true -- already set explicitly
• module: "Node16" -- still valid, not deprecated
• moduleResolution: "Node16" -- still valid, not deprecated
• target: "ES2020" -- still valid (only es3/es5 deprecated)
• esModuleInterop: true -- already set
• isolatedModules: true -- still valid
• No import ... assert {} syntax found
• No module Foo {} legacy syntax found
• No downlevelIteration usage
• No outFile usage
• No parameter properties used
• tsx (esbuild-based) -- independent of TS compiler, no risk

6. Recommended Migration Strategy

1. Wait for ecosystem: ts-jest and TypeDoc are blockers. Monitor their TS 6.0 support progress.
2. Run ts5to6: Use the official migration tool to handle baseUrl removal and rootDir inference changes automatically across all ~150 tsconfig files.
3. Add types arrays: Add "types": ["node"] to tsconfig.base.json and ensure test configs include "jest".
4. Upgrade typescript-eslint: Once the TS 6.0 PR merges (likely around the March 17 GA).
5. Test @ts-bridge/cli: Run a build with TS 6.0-beta to see if it works. File an issue / fix if needed.
6. Consider ts-jest alternatives: If ts-jest doesn't support TS 6.0 in time, evaluate @swc/jest as a drop-in replacement.
7. Use "ignoreDeprecations": "6.0": As a temporary escape hatch for baseUrl and any other deprecations during the transition period.