Migrating a JavaScript File

JS to TS in four moves

Because every JavaScript file is nearly valid TypeScript, migration is incremental:

1. Rename file.js to file.ts. It mostly compiles as is
2. Annotate parameters, the compiler flags each implicit any for you under strict mode
3. Extract interfaces for the object shapes flowing through the code, like Unit 3
4. Chase the remaining errors, most are real latent bugs being surfaced

Here is step 3 in action. The JavaScript version passed bare objects around, the TypeScript version names the shape:

interface Item {
  name: string;
  qty: number;
}

function receipt(items: Item[]): string

Teams migrate large codebases file by file this way, TypeScript and JavaScript coexist in one project while it happens.