Updated June 2026

Migrating JavaScript to TypeScript

Class Duration

14 hours of live training delivered over 2 days.

Student Prerequisites

Professional JavaScript development experience
Basic TypeScript familiarity (TypeScript Essentials or equivalent self-study)
Experience working in an existing production codebase
Familiarity with npm tooling and CI pipelines

Target Audience

This course is designed for developers and tech leads responsible for moving existing JavaScript codebases to TypeScript. It suits teams maintaining long-lived applications, platform engineers planning a monorepo-wide migration, and engineering managers who need a realistic strategy with measurable progress — not a rewrite. Participants leave with a concrete, incremental migration plan they can apply immediately.

Description

Migrating JavaScript to TypeScript is a strategy-and-execution course for teams that need to adopt TypeScript in an existing codebase without stopping feature work. It opens with the business case and the migration strategies that actually succeed, then moves into incremental adoption mechanics: enabling allowJs and checkJs, using JSDoc annotations as an on-ramp, and converting files in dependency order. Participants learn the strictness-ratcheting approach — start loose, then enable strict flags one at a time — so the migration delivers value early and never blocks the team.

The second day tackles the hard parts: typing legacy patterns such as dynamic objects, monkey patching, and CommonJS interop; managing third-party types with @types packages, declaration merging, and module augmentation; and migrating monorepos with project references. The course gives substantial attention to automation — codemods and conversion tooling, and AI-agent-assisted migration, where coding agents convert and verify modules at scale with the type checker in the loop as the reviewer that keeps generated changes honest. (For deeper coverage of agentic workflows, see our AI-Assisted Development with JavaScript and TypeScript course.) Throughout, participants learn to measure progress objectively and keep CI green at every step.

Learning Outcomes

Articulate the business case for TypeScript migration and choose a strategy suited to the codebase
Configure allowJs and checkJs to type-check JavaScript before converting it
Use JSDoc annotations as a low-friction on-ramp to typed code
Plan file conversion order using dependency analysis
Apply strictness ratcheting: migrate loose first, then enable strict flags one at a time
Type legacy patterns including dynamic objects, monkey patching, and CommonJS/ESM interop
Manage third-party types with @types packages, declaration merging, and module augmentation
Migrate monorepos incrementally using project references
Apply codemods and automated tooling to accelerate mechanical conversion
Run AI-agent-assisted migration with type-checker-in-the-loop verification and human review
Define and track migration metrics that show real progress
Keep CI green throughout the migration without freezing feature development

Training Materials

Comprehensive courseware is distributed online at the start of class. All students receive a downloadable MP4 recording of the training.

Software Requirements

Node.js 24 LTS (or current Node.js 22+)
Visual Studio Code or another TypeScript-aware editor
Permission to install npm packages and editor extensions
Git and a free GitHub account for lab repositories
Access to an AI coding agent for the agent-assisted labs (a course-provided option is available)

Training Topics

Migration Strategy and Business Case

What TypeScript buys an existing codebase: defects, refactoring, onboarding
Costs and risks: timeline, ecosystem, team skills
Big bang versus incremental: why incremental wins
Scoping: which code to migrate first and which to leave alone
Setting expectations with stakeholders

Incremental Adoption Mechanics

Adding TypeScript to a JavaScript build without breaking it
allowJs: mixing .js and .ts files
checkJs: type-checking JavaScript in place
JSDoc annotations as an on-ramp: @param, @returns, @type, @typedef
Converting files in dependency order
Coexistence with existing bundlers and test runners

Strictness Ratcheting

Starting loose: minimal compiler options that build
Enabling strict flags one at a time: noImplicitAny, strictNullChecks, and beyond
TypeScript 6 strict-by-default and what it means for new configs
Ratchet tooling: preventing new violations while old ones burn down
Suppression hygiene: @ts-expect-error over @ts-ignore

Typing Legacy Patterns

Dynamic objects: index signatures, Record, and discriminated alternatives
Monkey patching and global augmentation
CommonJS interop: esModuleInterop and default import pitfalls
Untyped event buses and string-keyed APIs
any quarantine: containing the untypeable

Third-Party Types

DefinitelyTyped and @types packages
Libraries that ship their own types
Writing local declaration files for untyped dependencies
Declaration merging and module augmentation
Pinning and upgrading type packages safely

Monorepo Migration with Project References

Composite projects and build orchestration
Migrating package by package
Sharing types across packages during a partial migration
Incremental builds and CI caching

Codemods and Automated Tooling

What codemods do well: mechanical, repetitive transforms
Conversion tooling: ts-migrate-style pipelines and jscodeshift
Limits of automation and where humans must review

AI-Agent-Assisted Migration

Using coding agents to convert and verify modules at scale
Type-checker-in-the-loop: the compiler as the agent's reviewer
Prompting patterns: one module at a time, tests must pass
Reviewing agent output: trust but verify
Going deeper: our AI-Assisted Development with JavaScript and TypeScript course

Measuring Progress and Keeping CI Green

Metrics: typed-file percentage, any count, strict-flag coverage
Dashboards and ratchet reports
CI gates that fail on regression, not on legacy debt
Shipping features mid-migration without conflict pain