When migrating from JS (or Flow) to TypeScript, you’ll likely make concessions for the initial conversion.

For instance, you might set noImplicitAny to false.

Once the migration is complete, you’ll want to enable this flag, but with a large codebase, new code gets added without types and now you have implicit anys everywhere with thousands of errors related to noImplicitAny!

So how do we enable noImplicitAny and get back to the "strict": true promise land?

Stem the Tide

We need to prevent new code from being written that errors with noImplicitAny.

Turns out it’s tricky (impossible?) to enable a TypeScript compiler option for only select files.

So instead I tried creating a separate tsconfig with noImplicitAny enabled and gradually adding files to it, but this doesn’t work. When you include a file you need to include all its dependencies which invariably pulls in most of the codebase and all those files need to abide by noImplicitAny. Back to square one.

So a separate tsconfig doesn’t work.


Here’s what I did instead but it’s a stop gap that supports enabling noImplicitAny.

Use @typescript-eslint/typedef:

  "rules": {
    "@typescript-eslint/typedef": [
        "parameters": true,
        "arrows": true,
        "memberVariableDeclaration": true,
        "propertyDeclaration": true

and then all function definition params and type definitions need to have types.

The side effect is that even when TypeScript could infer a parameter type, like with callback functions, you still need to provide an explicit type so you end up with more verbose code, but it’s only temporary until you can enable noImplicitAny.


You can use @typescript-eslint/typedef as a stopgap to get to noImplicitAny.