experience-lwc-typescript-migrate

Converting LWC to TypeScript

Convert a Lightning Web Component bundle from JavaScript to TypeScript. The deliverable is a fully-typed .ts implementation plus a .d.ts file that only exposes @api members (the public surface other LWCs consume).

When to Use This Skill

• User wants to migrate a single component or a folder of components from .js to .ts.
• User needs a .d.ts for an existing LWC so other components (or an external TypeScript host) can import it safely.
• User is adding type annotations to an already-renamed .ts LWC that hasn't been properly typed yet.
• User wants JSDoc-style type hints upgraded to real TypeScript types.

Prerequisites

• The component builds and runs correctly in JavaScript today.
• git is available (the rename must preserve history via git mv).
• A TypeScript compiler is wired into the build (either the SFDX TS pipeline or a standalone tsc step).

Workflow

Step 1 — Read the component

Open every file in the bundle:

componentName/
├── componentName.js
├── componentName.html
├── componentName.css
└── (possibly) __tests__/, __utam__/, existing .d.ts

Understand:

• What extends LightningElement? What is the class name?
• Which fields and methods carry the @api decorator?
• Which properties/methods have existing JSDoc (use as a type hint starting point, but validate against actual usage — JSDoc lies).
• Which parameters / return types can you infer from how the code is called internally?

Step 2 — Rename .js → .ts using git mv

git mv componentName/componentName.js componentName/componentName.ts

Repeat for any helper .js files in the bundle (unless they're already .ts). Never plain mv — that loses the history link TypeScript reviewers rely on.

Step 3 — Add type annotations in the .ts

Apply types in this priority order so you stop as soon as the public contract is solid:

1. @api properties and methods first. Generate JSDoc if it's missing, then translate JSDoc types to TS syntax (string, number, boolean, Promise<T>). Validate each JSDoc claim against the code before trusting it.
2. Complex shapes become interface or type aliases — not inline shapes repeated everywhere.
3. Optional members use ? only when the value is genuinely allowed to be undefined. Do not sprinkle ? defensively.
4. Private/internal state — still type it, but don't export the types. Use private for members that must never be touched by consumers.
5. Event handlers — prefer precise DOM event types:
   • MouseEvent for onclick (and other click-like handlers). click is dispatched as a MouseEvent — including keyboard-activated clicks — so typing it as PointerEvent would let handlers rely on pointer-only fields (pointerType, pressure, etc.) that are undefined in those cases.
   • PointerEvent for onpointerdown / onpointerup / onpointermove and other pointer* handlers where pointer-specific fields are actually meaningful.
   • CustomEvent<{ detail: ... }> for LWC custom events.
   • Event is the last resort; document why when using it.
6. Async methods always return Promise<T> — never bare T.
7. Avoid any. If you genuinely can't type something, use unknown and narrow with a type guard.

Reference patterns

Load [[assets/type-patterns.ts|assets/type-patterns.ts]] as an inline example covering property types, method types, and event handler types.

Step 4 — Generate the .d.ts

Create componentName.d.ts next to the .ts. It must:

• Contain only @api members — no private state, no internal methods, no lifecycle hooks unless they are themselves @api.
• Preserve @api JSDoc verbatim (including @type, @required, @default, @param, @returns tags) directly above each declaration.
• Declare the LWC module namespace c/componentName (or the org's namespace if different).

Template: load [[assets/dts-template.ts|assets/dts-template.ts]] as the starting .d.ts shape.

If the component has no @api members, still produce the module declaration with a comment explaining there's no public surface — don't skip the file.

Step 5 — Compile and test

• Run the TypeScript compiler (tsc --noEmit or the build's equivalent). Resolve every error before calling it done; no @ts-ignore patches.
• Run the component's existing Jest tests. The behavior should be identical.
• Run the bundled consumer-finder unconditionally — empty output is a valid result, not a reason to skip. The script resolves the search paths from sfdx-project.json's packageDirectories (or falls back to <project-root>), rejects any entry that escapes the project root, and performs the LWC-import search internally so the invocation is fully deterministic:

"<skill_dir>/scripts/find-consumers.sh" "<project-root>" "<componentName>"

For each match, confirm the consumer's expected types still align with the new .d.ts public surface.

Step 6 — Expected final bundle shape

componentName/
├── componentName.ts          # Main TypeScript implementation
├── componentName.html        # Template (unchanged)
├── componentName.css         # Styles (unchanged)
└── componentName.d.ts        # Type definitions (new)

Verification Checklist

Before conversion:

• [ ] Component is valid JS and all tests pass.
• [ ] You've identified every @api member and its intended type.

After conversion:

• [ ] git mv was used so history is preserved.
• [ ] Every variable and parameter in the .ts has a concrete type (no implicit any).
• [ ] Complex object shapes live in interface / type aliases, not inline repeats.
• [ ] Optional ? is only on genuinely optional fields.
• [ ] .d.ts exists, declares c/componentName, extends LightningElement, includes only @api members.
• [ ] Every @api JSDoc is preserved verbatim in the .d.ts.
• [ ] tsc passes with zero errors; no @ts-ignore or any used as a workaround.
• [ ] Jest tests still pass.

Common Pitfalls

• Using any to silence errors. Solve the actual type instead. If the value is truly unknown, use unknown + a type guard.
• Including private members in the .d.ts. The .d.ts is the public contract. Internal lifecycle and helpers must not leak.
• Losing JSDoc during the rename. Scan before and after — JSDoc comments on @api members must appear in both the .ts and .d.ts.
• Skipping git mv. Makes review miserable and confuses blame.
• Forgetting async return types. foo() with an async keyword always returns a Promise. Declare it.
• Typing onclick as PointerEvent. click is a MouseEvent (keyboard-triggered clicks included), so PointerEvent fields like pointerType are undefined for those events. Type onclick as MouseEvent; reserve PointerEvent for onpointer* handlers. Use MouseEvent | TouchEvent only when the code branches on TouchEvent distinctly.

Support Resources

• LWC TypeScript docs
• TypeScript Handbook
• LWC Developer Guide