How to Upgrade to TypeScript without anybody noticing, Part 2

This guide will show you how to fix Typescript compile errors in Javascript project that recently added Typescript support via a tsconfig.json. It assumes that the tsconfig.json is configured according to the description in part 1 of this post, and that you also installed types for some of your dependencies from the @types/* namespace. This guide is more of a list of tasks that you can pick and choose from, depending on what you want to fix first. Here are the tasks:

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.

This guide does not teach you how to write type definitions. The Declaration section of the Typescript handbook is the best place to learn about that. Here, you’ll just see types presented without a lot of explanation.

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 @types/shelljs doesn’t even include shelljs/make.d.ts right now. It’s completely missing. If you look at the source, shelljs/make.js does two things:

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

Let’s say you want to add make to Definitely Typed so that it is available in @types/shelljs. Your first step is to create make.d.ts inside node_modules/@types/shelljs/. This is the wrong location — it’s inside your own node_modules folder — but it makes development super easy to test that you’re actually adding the missing stuff. You can create a proper PR after everything is working.

Start with this:

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

This copies all of shelljs’ contents into the global namespace. Then 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 Definitely Typed 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. Types don’t exist at runtime, so the import will fail at runtime when Identifier is not found. 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 you’ll have to define them yourself. To start, define the typedefs you need as any. This gets rid of the errors at the cost of accuracy:

/** @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 you’ll see 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 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.

Let’s 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, change the typedefs one by one to ‘unknown’, and look for errors that pop up. For example:

/** @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 change TSTypeQuery from unknown to { left: unknown }. There’s no more indication of what the type of left is, so leave it as unknown:

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

As you can see, bottom-up type discovery can be a bit unsatisfying and underspecified, but it’s less disruptive to existing code. 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 you can do:

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 pompous name for “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, you can look at pattern-visitor.js in eslint to find the names of the constructor parameters. Then, a little lower in the Identifier method of OriginalPatternVisitor there is a usage of callback that gives enough information to guess its type.

Here’s what you’ll end 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) {

The right fix is to change 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) {

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.

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