How to add types to javascript with typescript

What is this post about?

Do you ever wanted to have a look at what typescript can do for your codebase without having to switch it all to .ts ?

If so, this post is for you! This tutorial will show how you can use typescript’s compiler to check your javascript code and how to use global type definitions to describe your types.

TL;DR

Let’s do it!

To start, on a new folder, we create a package.json file:

{ "name" : "js-types-tutorial" , "version" : "1.0.0" , "main" : "src/index.js" , "license" : "MIT" }

And add your dependencies using your package manager (in this case yarn ):

yarn add -D typescript

Create a new src/index.js :

let myVar = 3 myVar = " string "

The intention here is to get the following error when checking:

src/index.js:2:1 - error TS2322: Type '"string"' is not assignable to type 'number' .

To do so we update our package.json file:

{ "name" : "types" , "version" : "1.0.0" , "main" : "src/index.js" , "license" : "MIT" , "devDependencies" : { "typescript" : "^3.7.5" }, "scripts" : { "check" : "tsc -p jsconfig.json" } }

Note that we’ve add scripts.check and installing typescript added devDependencies.typescript . The check script invokes tsc , the TypeScrip Compiler and tell it to look for the -p roject configuration at jsconfig.json .

Which leeds us to: jsconfig.json

{ "include" : [ "src" ], "compilerOptions" : { "checkJs" : true , "noEmit" : true } }

A list of things going on here:

include : tells tsc where the code is

: tells where the code is compilerOptions.checkJs : tells tsc to look for .js too, instead of only .ts

: tells to look for too, instead of only compilerOptions.noEmit : This one is trickier. It means that the compiler will parse the files, do the syntatic and the semantic analisys and halt without write any compiled files.

Now you should be able to run npm run check and see this output:

> types@1.0.0 check > tsc -p jsconfig.json src/index.js:2:1 - error TS2322: Type '"string"' is not assignable to type 'number' . 2 myVar = "string" ~~~~~ Found 1 error.

Bingo! We got type-checking for .js files!

So far, so good. But what happens with we add dependencies?

To test our setup, let’s try to solve a small problem. Imagine we want to create a cli tool that takes one argument, read the process.stdin and print the number of lines that contained that one argument. (which would be equivalent to grep -e myWord my-file.txt | wc -l )

Let’s create a file to handle the filesystem, I called it src/stdin.js :

import * as fs from ' fs ' export function readStdIn () { return fs . readFileSync ( ' /dev/stdin ' ). toString () }

And editing our src/index.js :

import { filter , includes , length , pipe , split } from ' ramda ' import { readStdIn } from ' ./stdin ' if ( process . argv . length < 3 ) { console . error ( ' please pass in the word to count ' ) process . exit ( - 1 ) } const inputFile = readStdIn () const argument = process . argv [ 2 ] pipe ( split ( '

' ), filter ( includes ( argument )), length , console . log )( inputFile )

Checking the project again:

 npm run check > tsc -p jsconfig.json src/index.js:1:55 - error TS2307: Cannot find module 'ramda' . 1 import { filter, includes, length, pipe, split } from 'ramda' ~~~~~~~ src/index.js:4:5 - error TS2580: Cannot find name 'process' . Do you need to install type definitions for node? Try ` npm i @types/node ` . 4 if ( process.argv.length < 3 ) { ~~~~~~~ src/index.js:6:3 - error TS2580: Cannot find name 'process' . Do you need to install type definitions for node? Try ` npm i @types/node ` . 6 process.exit ( -1 ) ~~~~~~~ src/index.js:10:18 - error TS2580: Cannot find name 'process' . Do you need to install type definitions for node? Try ` npm i @types/node ` . 10 const argument = process.argv[2] ~~~~~~~ src/stdin.js:1:21 - error TS2307: Cannot find module 'fs' . 1 import * as fs from 'fs' ~~~~ Found 5 errors.

Wow, that was a lot. First thing, the errors says we should add type definitions for node, let’s try it:

yarn add -D @types/node

And try check again:

 npm run check > tsc -p jsconfig.json src/index.js:1:55 - error TS2307: Cannot find module 'ramda' . 1 import { filter, includes, length, pipe, split } from 'ramda' ~~~~~~~ Found 1 error.

Way better, huh? Now we add ramda

yarn add ramda

But if we run check again we’ll run into ~25 errors like this:

node_modules/ramda/src/view.js:22:12 - error TS2304: Cannot find name 'Lens'. 22 * @param {Lens} lens

What is missing? type definitions! Install it using yarn add -D @types/ramda and check will succeds.

Testing the cli

So we wrote and type-checkd our cli, lets use it.

cat jsconfig.json | node . true # we expect see 1 as output as jsconfig.json has one line that include "true". index.js:1 ( function ( exports, require, module, __filename, __dirname ) { import { filter, includes, length, pipe, split } from 'ramda' ^^^^^^ SyntaxError: Unexpected token import at createScript ( vm.js:80:10 ) at Object.runInThisContext ( vm.js:139:10 ) at Module._compile ( module.js:616:28 ) at Object.Module._extensions..js ( module.js:663:10 ) at Module.load ( module.js:565:32 ) at tryModuleLoad ( module.js:505:12 ) at Function.Module._load ( module.js:497:3 ) at Function.Module.runMain ( module.js:693:10 ) at startup ( bootstrap_node.js:191:16 ) at bootstrap_node.js:612:3

Ops, it seems we are using unsupported syntax for javascript. We can solve it either by using node v13 or by transpiling.

Luckily tsc is a transpiler and we can easily configure it to transpile our .js files:

jsconfig.json

{ "include" : [ "src" ], "compilerOptions" : { "checkJs" : true , "declaration" : true , "noEmit" : false , "outDir" : "dist" , "module" : "commonJS" } }

To better describe what tsc will be doing you can rename check script to build or compile . We also update the main key of package.json to "dist/index.js"

 npm run compile # OK > tsc -p jsconfig.json  cat jsconfig.json | node . true # expected 2 2  tree dist dist ├── index.d.ts ├── index.js ├── stdin.d.ts └── stdin.js 0 directories, 4 files

As you can see, tsc created four files under dist dir. One transpiled .js for each .js in our src and two .d.ts files within generated type definitions.

dist/stdin.d.ts

export function readStdIn (): string ;

TL;DR

To wrap up what we’ve learned

we create a new javascript project used jsconfig.json to configure our project imported typescript and created a npm script that uses tsc -p jsconfig.json imported @types/node , ramda @types/ramda created a small cli and compiled it using that npm script

I hope this tutorial was helpfull. You can reach me out at @munizart and @hurricanecart.

Thank you for reading.