Recently I started using TypeScript (TS) with React Native. Now I won’t be going over the benefits of typescript in this article there are plenty for other articles that will explain the benefits (and drawbacks).

TS is a superset of JavaScript (JS) so anything JS can do TS can do (and more). One of the main advantages of TS is it’s strict type checking. JS is weakly typed which means variable and parameters can be of any type. One of the major downsides of this approach is in larger projects it can make code harder to follow and more bug prune. For example, if you’re expecting a variable to be an integer but turns out to be a string. Typescript makes bugs like this much easier to catch because it is strongly typed and each variable and parameter is given a type. Lets say you have the following function.

add = (x, y) => {
  return x + y;

Now we expect x and y to be integers here of course however we are not checking types so let’s say we did the following.

add(2, 3) === 5; // true
add(2, "3") === "23"; // true
add("2", "3") === "23"; // true

As you can see if you accidentally passed a string to add it returns a result we don’t expect. TS helps us catch theses types of errors. The following would be the equivalent functions add() written in TS.

add = (x: number, y: number) => {
  return x + y;

Definitely Typed

When using JS libraries not written in TS we need a file which stores the type definitions of functions and their parameters this is referred to as the global type definition file. Lots of popular libraries already have this defined in a huge project on GitHub called DefinitelyTyped. You can actually add these to your project by using yarn add @types/<package_name>.

This repo is huge and has over 5,000 libraries already defined however for more obscure projects you may have to write you’re own definitions. This then means we can take full advantage of TS even with any external libraries we use. In this article, we will write definitions for react-native-canvas.

  1. Fork the DefinitelyTyped project on GitHub, how to fork on GitHub.

  2. Git clone the project onto your computer, like so git clone [email protected]:hmajid2301/DefinitelyTyped.git.

  3. Open the project in your favourite text editor and run the following commands in the root (project) directory.

  4. Execute the following command using either yarn or npm, replace react-native-canvas with your package name. Before you run the command you should make sure the package doesn’t exist in which case all you likely need to do is update its type definitions

  5. You should see a new folder with the package name in the types folder.

yarn npx dts-gen --dt --name react-native-canvas --template module

# or

npm install
npm npx dts-gen --dt --name react-native-canvas --template module


You should now have four auto-generated files, we can leave tslint.json as it is. Since this a React Native library we will have to edit tsconfig.json with some new parameters. If you’re confused you can take a look at other type packages to see how they’ve changed the tsconfig file. There are plenty of React Native examples to take a look at. The tsconfig now looks like this

  "compilerOptions": {
    "module": "commonjs",
    "lib": ["es6"],
    "noImplicitAny": true,
    "noImplicitThis": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "baseUrl": "../",
    "typeRoots": ["../"],
    "types": [],
    "noEmit": true,
    "forceConsistentCasingInFileNames": true,
    "jsx": "react-native"
  "files": ["index.d.ts", "react-native-canvas-tests.tsx"]


Now onto the main file to edit index this contains the types for the library. So now we will have to look at the library itself and take a look at the functions components etc. If the index.d.ts file has been created properly at the top in comments you should see something like this.

// Type definitions for react-native-canvas 0.1
// Project:
// Definitions by: hmajid2301 <>
// Definitions:
// TypeScript Version: 3.1

The first two lines are auto-generated, the next line I added my name and the URL to my GitHub account. The following line is also auto-generated and the final line is required because we are defining our types with react-native-canvas.

Now we actually need to look at the react-native-canvas library, so we know how to define our types correctly. The source code is in the folder src, now the first class I use is Canvas. Here is a small snippet of the source code.

export default class Canvas extends Component {
  static propTypes = {
    style: PropTypes.shape(ViewStylePropTypes),
    baseUrl: PropTypes.string,
    originWhitelist: PropTypes.arrayOf(PropTypes.string),

The main thing I am interested in is the props we will need to define these in the index.d.ts file. So here we have a React Native component class export default class Canvas extends Component, in the index.d.ts file this will become export default class Canvas extends React.Component<CanvasProps> in this class, we don’t have any state if we did then it would look like export default class Canvas extends React.Component<CanvasProps, StateProps>.

Now we’ve defined our class lets define our props we will define our props as an interface called CanvasProps which will be defined like so.

export interface CanvasProps {
  style?: StyleProp<ViewStyle>;
  baseUrl?: string;
  originWhitelist?: string[];
  ref: (canvas: Canvas) => any;

The first objects are the same as the first three prop types in the original JS library. They are defined almost exactly the same bar some syntax differences, in JS style: PropTypes.shape(ViewStylePropTypes) as a pose to style?: StyleProp<ViewStyle> in TS. However in the original, the ref prop is not defined, so we define it ourselves for completeness, ref: (canvas: Canvas) => any. In this case, the ref prop takes an input of type Canvas and can return anything. Below is an example of ref being used (in JS).

class App extends Component {
  handleCanvas = (canvas) => {
    const ctx = canvas.getContext("2d");
    ctx.fillStyle = "purple";
    ctx.fillRect(0, 0, 100, 100);

  render() {
    return <Canvas ref={this.handleCanvas} />;

In our Canvas class, we have to define our properties, according to the documentation we have the following functions/attributes.

  • Canvas#height
  • Canvas#width
  • Canvas#getContext()
  • Canvas#toDataURL()

These get defined as follows;

width: number;
height: number;
toDataURL: () => string;
getContext: (context: string) => CanvasRenderingContext2D;

This should all be pretty straight forward, the final property getContext returns CanvasRenderingContext2D. This another interface we define using the CanvasRenderingContext2D.js class (separate file in src folder). It’s quite a long interface so if you want to see it here.

We then repeat this process for the remaining classes, Image, ImageData which look like follows. In these classes, we also define the constructor, which just contains the arguments and the type of object they expect. Note that these classes aren’t React Native components so we define them as normal classes. We also give them named exports i.e. export class Image rather than export default class Image, this is because this is how they are defined in the react-native-canvas library.

export class Image {
  constructor(canvas: Canvas, height?: number, width?: number);
  crossOrigin: string | undefined;
  height: number | undefined;
  width: number | undefined;
  src: string | undefined;
  addEventListener: (event: string, func: (...args: any) => any) => void;

export class ImageData {
  constructor(canvas: Canvas, data: number[], height: number, width: number);
  readonly data: number[];
  readonly height: number;
  readonly width: number;

The final class to define is Path2D, which looks like

export class Path2D {
  constructor(canvas: Canvas, ...args: any);
  addPath: (
    path: Path2D,
    transform?: {
      a: number;
      b: number;
      c: number;
      d: number;
      e: number;
      f: number;
  ) => void;

  closePath: CanvasRenderingContext2D["closePath"];
  moveTo: CanvasRenderingContext2D["moveTo"];
  lineTo: CanvasRenderingContext2D["lineTo"];
  bezierCurveTo: CanvasRenderingContext2D["bezierCurveTo"];
  quadraticCurveTo: CanvasRenderingContext2D["quadraticCurveTo"];
  arc: CanvasRenderingContext2D["arc"];
  arcTo: CanvasRenderingContext2D["arcTo"];
  ellipse: CanvasRenderingContext2D["ellipse"];
  rect: CanvasRenderingContext2D["rect"];

Again this class is very similar to the classes defined above except some of the properties look like closePath: CanvasRenderingContext2D["closePath"]. This is because closePath shares the same definition as closePath in CanvasRenderingContext2D, which is defined as closePath: () => void. So rather than define it twice we just copy the definition in CanvasRenderingContext2D.


This is where we define some tests how the library should be used and their props types.

import * as React from "react";
import { View } from "react-native";
import Canvas, {
    Image as CanvasImage,
} from "react-native-canvas";

class CanvasTest extends React.Component {
    render() {
        return (
                <Canvas ref={this.handleCanvas} />

So we import our library then we render our Canvas component.

handleCanvas = (canvas: Canvas) => {
  canvas.width = 100;
  canvas.height = 100;

  const context = canvas.getContext("2d");
  context.fillStyle = "purple";
  context.fillRect(0, 0, 100, 100);

  const ellipse = new Path2D(canvas);
  ellipse.ellipse(50, 50, 25, 35, (45 * Math.PI) / 180, 0, 2 * Math.PI);
  context.fillStyle = "purple";

  const image = new CanvasImage(canvas);
  canvas.width = 100;
  canvas.height = 100;

  image.src =
  image.addEventListener("load", () => {
    context.drawImage(image, 0, 0, 100, 100);

  const imageData = context.getImageData(0, 0, 100, 100);
  const data = Object.values(;
  const length = Object.keys(data).length;
  for (let i = 0; i < length; i += 4) {
    data[i] = 0;
    data[i + 1] = 0;
    data[i + 2] = 0;
  const imgData = new ImageData(canvas, data, 100, 100);
  context.putImageData(imgData, 0, 0);

Then in handleCanvas, we test out the different classes we defined, include Canvas, ImageData, Image and Path2D and that’s it. The above example is taken from a few examples in example/App.js within react-native-canvas. Ok now we’ve defined our files lets make sure the pull request (PR) will be accepted let’s run yarn run lint react-native-canvas. If the linter doesn’t complain then we can commit and push our changes to our GitHub fork and make PR.