How to Upgrade to Typescript Without Anybody Noticing, Part 1

This guide will show you how to upgrade to TypeScript without anybody noticing. Well, people might notice — what I really mean is that you won’t have to change your build at all. You’ll have the ability to get errors and completions in supported editors and to get errors on the command line from tsc, the TypeScript compiler, but you won’t have to integrate TypeScript into your build.

Here’s what you’ll actually need to check in to source control:

1. A TypeScript configuration file, tsconfig.json.
2. New dev dependencies from the @types package.
3. A TypeScript declaration file to hold miscellaneous types.

How can you upgrade with so little change? Well, the secret is that you’re not using TypeScript. The TypeScript compiler can check Javascript just fine, so you can stick with Javascript and use JSDoc to provide type information. This is less convenient than TypeScript’s syntax for types, but it means that your files stay plain old Javascript and your build (if any) doesn’t change at all.

Let’s use TypeScript-eslint-parser as an example package so you can follow along if you want. Confusingly, even though the name includes “TypeScript”, the parser is actually written in Javascript, although it has since been merged into a larger project that is written in TypeScript.

This guide assumes that you have used TypeScript enough to:

1. Have an editor set up to work with TypeScript.
2. Have used npm to install a package.
3. Know the basic syntax of type annotations — number, { x: any }, etc.

If you want to look at the package after the upgrade, you can run the following commands, or take a look at the branch on github:

git clone https://github.com/sandersn/TypeScript-eslint-parser
cd TypeScript-eslint-parser
git checkout add-tsconfig
npm install

Also make sure that you have TypeScript installed on the command line:

npm install -g TypeScript

Add tsconfig

Your first step is to start with tsc --init and change the settings in the tsconfig.json that it produces. There are other ways to get started, but this gives you the most control. Run this command from the root of the project:

tsc --init

Here is what you should end up with, skipping the lines you don’t need to change:

{
  "compilerOptions": {
    "allowJs": true,
    "checkJs": true,
    "noEmit": true,
    "target": "esnext",
    "module": "commonjs",
    "resolveJsonModule": true,
    "strict": false
  },
  "exclude": [
    "tests/fixtures/",
    "tests/integration/"
  ]
}

For Javascript, your “compilerOptions” will pretty much always look like this.

• allowJs — Compile JS files.
• checkJs — Give errors on JS files.
• noEmit — Don’t emit downlevel code; just give errors.
• target — Target the newest version of EcmaScript since we’re not emitting code anyway.
• module — Target node’s module system since we’re not emitting code anyway.
• resolveJsonModule — Compile JSON files (if they’re small enough).
• strict — Don’t give the strictest possible errors.

A few notes:

• "target" and "module" should not actually matter since they have to do with generated downlevel code, but you’ll get some bogus errors if you use ES6 classes like Map or Set.
• "resolveJsonModule" is optional, but you’ll need it if your code ever requires a JSON file, so that TypeScript will analyze it too.
• "strict" should be false by default. You can satisfy the compiler on strict mode with pure Javascript, but it can require some odd code.

You may want to specify which files to compile. For TypeScript-eslint-parser, you’ll be happiest with an "exclude" list. You may want to check all the source files, which is what you get by default. But it turns out that checking a Javascript parser’s tests is a bad idea, because the tests are themselves malformed Javascript files. Those malformed test files shouldn’t be checked and mostly don’t parse anyway.

You might want to use "include" if, say, you only want to check your source and not your tests or scripts. Or you can use "files" to give an explicit list of files to use, but this is annoying except for small projects.

OK, you’re all set. Run tsc and make sure it prints out errors. Now open up files in your editor and make sure the same errors show up there. Below are the first few errors you should see:

Makefile.js(55,18): error TS2304: Cannot find name 'find'.
Makefile.js(56,19): error TS2304: Cannot find name 'find'.
Makefile.js(70,5): error TS2304: Cannot find name 'echo'.

You should be able to see the same errors when you open Makefile.js in your editor and look at lines 55 and 56.

Congratulations! You’ve done the only required part of the upgrade. You can check in tsconfig.json and start getting benefits from TypeScript’s checking in the editor without changing anything else. Of course, there are a huge number of errors, hardly any of which are due to real bugs. So the next step is to start getting rid of incorrect errors and improving TypeScript’s knowledge of the code.

Here’s the commit.

Install @types packages.

Your first order of business is to install types for packages you use. This allows TypeScript to understand them, which makes it the easiest way to reduce the number of errors. Basically, if you have a dependency on some package, say, jquery, and you see errors when you use it, you probably need a dev dependency on @types/jquery. The type definitions in @types/jquery give TypeScript a model of jquery that it can use to provide editor support, even though jquery was written before TypeScript existed.

Definitely Typed is the source for the packages in the @types namespace. Anybody can contribute new type definitions, but tons of packages already have type definitions, so you will probably find that most of your dependencies do too.

Here’s a good starting set for TypeScript-eslint-parser, although there are likely more available:

npm install --save-dev @types/node
npm install --save-dev @types/jest
npm install --save-dev @types/estree
npm install --save-dev @types/shelljs@0.8.0
npm install --save-dev @types/eslint-scope

After the installation, these three types packages still didn’t work (although notice that we intentionally installed an old version of shelljs – more on that later):

• @types/shelljs
• @types/eslint-scope
• @types/estree

They fail for different reasons, though. shelljs and es-lint-scope just don’t have types for a lot of their values. estree has all the correct types, but the types aren’t imported correctly.Part 2 shows how to fix these two problems.

At this point, you have types for some of your packages working and your own code checked by the TypeScript compiler. The next step is to fix compile errors in the rest of the package types, or to fix them in your own code. Or you can just ignore the errors and start using the TypeScript support in the editor.

Next up: Part 2, to learn about the various kinds of fixes.

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