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How to Upgrade to TypeScript Without Anybody Noticing

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 just 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.

I'm going to 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 actually written in Javascript.

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

I like 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

After some editing I ended up with this tsconfig.json:

{
  "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, I added an "exclude" list. I actually wanted 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 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 is 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 definitions, but tons of packages already have 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 I intentionally had to install an old version of shelljs -- more on that later):

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

They fail for different reasons, though. shelljs and es-lint-scope are just missing whole chunks of the package's types. estree has all the correct types, but the types aren't imported correctly.

So now you can decide where to start fixing errors.

You can

  1. Add missing types in dependencies.
  2. Fix references to types in dependencies.
  3. Add missing types in your own code.
  4. Work around missing types in dependencies.
  5. Fix errors in existing types.
  6. Add type annotations to everything else.

Add missing types in dependencies

Let's start with @types/shelljs. In Makefile.js, I see a few errors. The first is that the module require('shelljs/make') isn't found. The second group of errors is that the names 'find', 'echo' and a few others aren't found.

These errors are related. It turns out that shelljs doesn't even include shelljs/make right now. It's completely missing. I looked it up and shelljs/make does two things:

  1. Add a global object named 'target' that allows you to add make targets.
  2. Add the contents of the parent shelljs to the global scope.

So I want to add this to Definitely Typed. My first step is to create make.d.ts inside node_modules/@types/shelljs/. This is just for development, because it makes it super easy to test that I'm actually fixing the missing stuff.

I start with this:

import * as shelljs from './';
declare global {
  const cd: typeof shelljs.cd;
  const pwd: typeof shelljs.pwd;
  // ... all the rest ...
}

Then I add the type for 'target' to the globals as well:

const target: {
  all?: Target;
  [s: string]: Target;
}
interface Target {
  (...args: any[]): void;
  result?: any;
  done?: boolean;
}

This exposes a couple more errors. See the section on fixing errors in existing types for how to fix those.

Now we want to publish this to Definitely Typed:

  1. Fork DefinitelyTyped on github.
  2. git clone https://github.com/your-name-here/DefinitelyTyped
  3. cp node_modules/@types/shelljs/make.d.ts ~/DefinitelyTyped/types/shelljs/
  4. git checkout -b add-shelljs-make
  5. Commit the change and push it to your github fork.
  6. Create a PR for the change.

If there are lint problems, the CI run on Travis will catch them.

For more detail on writing definitions for Definitely Typed, see the Declaration section of the Typescript handbook.

Fix references to types in dependencies

typescript-eslint-parser actually has quite a bit of type information in its source already. It just happens to be written in JSDoc, and it's often almost, but not quite, what Typescript expects to see. For example, in analyze-scope.js, visitPattern has an interesting mix of types:

/**
 * Override to use PatternVisitor we overrode.
 * @param {Identifier} node The Identifier node to visit.
 * @param {Object} [options] The flag to visit right-hand side nodes.
 * @param {Function} callback The callback function for left-hand side nodes.
 * @returns {void}
 */
visitPattern(node, options, callback) {
    if (!node) {
        return;
    }

    if (typeof options === "function") {
        callback = options;
        options = { processRightHandNodes: false };
    }

    const visitor = new PatternVisitor(this.options, node, callback);
    visitor.visit(node);

    if (options.processRightHandNodes) {
        visitor.rightHandNodes.forEach(this.visit, this);
    }
}

In the JSDoc at the start, there's an error on Identifier. (Object and Function are fine, although you could write more specific types.) That's weird, because those types do exist in estree. The problem is that they're not imported. Typescript lets you import types directly, like this:

import { Identifier, ClassDeclaration } from "estree";

But this doesn't work in Javascript because those are types, not values. They don't exist at runtime, so the import will fail at runtime. Instead, you need to use an import type. An import type is just like a dynamic import, except that it's used as a type. So, just like you could write:

const estree = import("estree);

to dynamically import the Identifier type from estree, you can write:

/** @type {import("estree").Identifier */
var id = ...

to import the type Identifier without an import statement. And, because it's inconvenient to repeat import all over the place, you usually want to write a typedef at the top of the file:

/** @typedef {import("estree").Identifier} Identifier */

With that alias in place, references to Identifier resolve to the type from estree:

/**
 * @param {Identifier} node now has the correct type
 */

Here's the commit.

Add missing types in your own code

Fixing these types still leaves a lot of undefined types in analyze-scope.js. The types look like estree types, but they're prefixed with TS-, like TSTypeAnnotation and TSTypeQuery. Here's where TSTypeQuery is used:

/**
 * Create reference objects for the object part. (This is `obj.prop`)
 * @param {TSTypeQuery} node The TSTypeQuery node to visit.
 * @returns {void}
 */
TSQualifiedName(node) {
    this.visit(node.left);
}

It turns out that these types are specific to typescript-eslint-query. So we'll have to define them ourselves. To start, I defined a lot more typedefs with any. This gets rid of the errors:

/** @typedef {any} TSTypeQuery */
// lots more typedefs ...

At this point, you have two options: bottom-up discovery of how the types are used, or top-down documentation of what the types should be.

Bottom-up discovery, which is what I'll show below, has the advantage that you will end up with zero compile errors afterward. But it doesn't scale well; when a type is used throughout a large project, the chances of it being misused are pretty high.

Top-down documentation works well for large projects that already have some kind of documentation. You just need to know how to translate documentation into Typescript types — the the Declaration section of the Typescript handbook is a good starting point for this. You will sometimes have to change your code to fit the type using the top-down approach as well. Most of the time that's because the code is questionable and needs to be changed, but sometimes the code is fine and the compiler gets confused and has to be placated.

I'm going to use bottom-up discovery in this case because it looks like top-down documentation would involve copying the entire Typescript node API into analyze-scope.js. To do this, I changed the typedefs one by one to 'unknown', and went looking for errors that popped up:

/** @typedef {unknown} TSTypeQuery */

Now there's an error is on the usage of TSTypeQuery in TSQualifiedName, node.left:

/**
 * Create reference objects for the object part. (This is `obj.prop`)
 * @param {TSTypeQuery} node The TSTypeQuery node to visit.
 * @returns {void}
 */
TSQualifiedName(node) {
    this.visit(node.left);
    //              ~~~~
    // error: type 'unknown' has no property 'left'
}

Looks like TSTypeQuery is supposed to have a left property, so I changed TSTypeQuery from unknown to { left: unknown }. I still don't know what the type of left is, so I'll leave it as unknown.:

/** @typedef {{ left: unknown }} TSTypeQuery */

Here's the commit.

Work around missing types

Sometimes you'll find that one of your dependencies has no @types package at all. You are free to define types and contribute them to Definitely Typed, of course, but you usually need a quick way to work around missing dependencies. The easiest way is to add your own typings file to hold workarounds. Let's look at visitPattern in analyze-scope.js again:

/**
 * Override to use PatternVisitor we overrode.
 * @param {Identifier} node The Identifier node to visit.
 * @param {Object} [options] The flag to visit right-hand side nodes.
 * @param {Function} callback The callback function for left-hand side nodes.
 * @returns {void}
 */
visitPattern(node, options, callback) {
    if (!node) {
        return;
    }

    if (typeof options === "function") {
        callback = options;
        options = { processRightHandNodes: false };
    }

    const visitor = new PatternVisitor(this.options, node, callback);
    visitor.visit(node);

    if (options.processRightHandNodes) {
        visitor.rightHandNodes.forEach(this.visit, this);
    }
}

Now there is an error on

const visitor = new PatternVisitor(this.options, node, callback)
                ~~~~~~~~~~~~~~~~~~
                Expected 0 arguments, but got 3.

But if you look at PatternVisitor in the same file, it doesn't even have a constructor. But it does extend OriginalPatternVisitor:

const OriginalPatternVisitor = require("eslint-scope/lib/pattern-visitor");
// much later in the code...
class PatternVisitor extends OriginalPatternVisitor {
    // more code below ...
}

Probably OriginalPatternVisitor has a 3-parameter constructor which PatternVisitor inherits. Unfortunately, eslint-scope doesn't export lib/pattern-visitor, so PatternVisitor doesn't get the 3-parameter constructor. It ends up with a default 0-parameter constructor.

As described in "Add missing types in dependencies", you could add OriginalPatternVisitor in lib/pattern-visitor.d.ts, just like we did for make.d.ts in shelljs. But when you're just getting started, sometimes you just want to put a temporary type in place. You can add the real thing later. Here's what I did:

  1. Create types.d.ts at the root of typescript-eslint-parser.
  2. Add the following code:
declare module "eslint/lib/pattern-visitor" {
    class OriginalPatternVisitor {
        constructor(x: any, y: any, z: any) {
        }
    }
    export = OriginalPatternVisitor;
}

This declares an "ambient module", which is a weaselly name for "my fake workaround module". It's designed for exactly this case, though, where you are overwhelmed by the amount of work you need to do and just want a way to fake it for a while. You can even put multiple declare modules in a single file so that all your workarounds are in one place.

After this, you can improve the type of OriginalPatternVisitor in the same bottom-up or top-down way that you would improve any other types. For example, I looked at pattern-visitor.js in eslint to find the names of the constructor parameters. Then I looked a little lower at the Identifier method of OriginalPatternVisitor to guess the type of callback.

Here's what I ended up with:

declare module "eslint-scope/lib/pattern-visitor" {
    import { Node } from "estree";
    type Options = unknown;
    class OriginalPatternVisitor {
        constructor(
            options: Options,
            rootPattern: Node,
            callback: (pattern: Node, options: Options) => void);
    }
    export = OriginalPatternVisitor;
}

Fix errors in existing types

Unfortunately, the improved type for pattern-visitor once again causes an error on new PatternVisitor. This time, the callback's type, Function isn't specific enough to work with the specific function type of the callback:

callback: (pattern: Node, options: Options) => void

So the existing JSDoc type annotation needs to change:

/**
 * Override to use PatternVisitor we overrode.
 * @param {Identifier} node The Identifier node to visit.
 * @param {Object} [options] The flag to visit right-hand side nodes.
 * @param {Function} callback The callback function for left-hand side nodes.
 * @returns {void}
 */
visitPattern(node, options, callback) {

I changed the type Function to the more precise (pattern: Node, options: Options) => void:

/**
 * Override to use PatternVisitor we overrode.
 * @param {Identifier} node The Identifier node to visit.
 * @param {Object} [options] The flag to visit right-hand side nodes.
 * @param {(pattern: Node, options: import("eslint-scope/lib/pattern-visitor").Options) => void} callback The callback function for left-hand side nodes.
 * @returns {void}
 */
visitPattern(node, options, callback) {

Except that Options isn't exported by my pattern-visitor workaround. So I moved it in the global namespace, outside the declare module:

Add JSDoc types to everything else

Once you get all the existing type annotations working, the next step is to add JSDoc types to everything else. You can turn on "strict": true to see how far you have to go — among other things, this marks any variables that have the type any with an error.

You should fall into a back-and-forth of adding new JSDoc type annotations and fixing old types. Usually old types just need to be updated to work with Typescript, but sometimes you'll find a bug.