I had the pleasure of migrating an existing codebase from straight nodejs to Typescript. Here’s a few of the headaches/gotchas I ran into.

Extending from Error breaks things

The following custom error class resulted in an error complaining about instanceof:

class ApiError extends Error {
    constructor(message: string) {
      this.name = 'ApiError';
    }
}

Microsoft explain the issue and point out a simple fix in their Breaking Changes list

Mocha not picking up sub directories with –recursive

With a test structure like:

test/controllers/book.spec.ts
test/middleware/log.spec.ts
test/app.spec.ts

running mocha --recursive test/**/*.spec.ts will only see mocha pickup tests in app.spec.ts, despite the --recursive option being set.

As ScottFreeCode points out the solution is to quote the glob to avoid having the shell interpret the **.

Eg. mocha --recursive "test/**/*.spec.ts"

Typescript compiler will not touch non-ts/js files

There’s no built in mechanism to have the compiler copy non-typescript/javascript files, such as .json schemas to the dist directory. This instead needs to be done separately.

With only a few files to copy I ended up creating a simple bin script to copy the files before running tsc.

Sequelize unexpected token in map file error

A fairly common pattern to follow when using Sequelize is to load all model files defined in a particular directory, with code like:

fs.readdirSync('models/').forEach((filename) => {
  ... load model
	models[model] = model;
});

This works fine with javascript, but will cause a runtime Unexpected token with Typescript when map file generation is turned on. Any code loading files should be modified to explicitly check the extension. In this case, we want to exclude any *.map.js files from being loaded.

More details can be found on this TypeScript github issue.

Cannot redeclare block-scoped variable

With a codebase split up amongst a number of files, Typescript instantly complained about Cannot redeclare block-scoped variable whenever attempting to load a local file.

The solution is to ensure each file exported every function/variable used in other files.

export function doSomething() {
  ...
}

export function doSomethingElse() {
  ...
}
module.exports = { doSomething, doSomethingElse};