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TypeScript Advanced: Types That Hold Up in Production
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TypeScript Advanced: Types That Hold Up in Production

Check Yourself: Migrate and Keep Moving

Five scenario questions on migrating a folder, allowJs and checkJs, and build performance. If you miss more than one, reread the lessons in this section.

5 min TypeScriptTesting and DebuggingPerformance OptimizationType SystemsPackaging and Dependencies Advanced
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1. What is the safest first step in migrating a JavaScript folder to TypeScript?
Turn on checkJs for the whole repository and fix all the errors first. Rename every file to .ts in one commit and fix the errors afterwards. Delete the tests until the migration is over. Make the build accept both file types with allowJs, and convert the files that import nothing from the folder first.
2. A file is renamed from discount.js to discount.ts. Another file under nodenext imports "./discount.js". What do you change in the import?
Change it to "./discount.d.ts". Change it to "./discount.ts". Nothing. The path names the output file, and the compiler finds discount.ts for it. Remove the extension.
3. What is a reasonable way to stop checkJs from blocking the build on day one?
Leave checkJs on and disable strict mode. Turn it on and add ts-nocheck to the files that fail, then count those files as the to-do list. Add ts-ignore above every error in the repository. Turn checkJs off and never look at the JavaScript.
4. Which JSDoc habits did the lesson show TypeScript 7 rejecting under checkJs?
Every @param tag with a type in braces. Comments that contain the word typedef. A Closure style function type, @enum used as a type, and a bare question mark as a type. The @returns tag.
5. After the first full run, how does incremental make later tsc runs faster, and what do you do with the file it writes?
It skips the type check completely and only emits JavaScript. It writes the output to a shared network folder. It saves what the compiler learned in a tsbuildinfo file and rechecks only the files that changed and what depends on them. Keep the file out of git and cache it in CI. It runs the compiler in the background between your edits.

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Five scenario questions on migrating a folder, allowJs and checkJs, and build performance. If you miss more than one, reread the lessons in this section.

Duration
5 min
Last update
10/08/2026
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