checkJs is the option that makes TypeScript report errors in JavaScript files. With it on, tsc and your editor check every .js file in the project the way they check .ts files, using your JSDoc comments where you have written them and inference everywhere else.
Every jsconfig.json on this site sets it. In TypeScript 5.9, checkJs also turns on allowJs when you leave allowJs out, so "checkJs": true on its own both includes your JavaScript files and checks them.
At a glance
| Write | "checkJs": true |
|---|---|
| On the command line | --checkJs |
| Type | boolean |
| Default in TypeScript 5.9 | false |
| TypeScript's description | Enable error reporting in type-checked JavaScript files. |
| In the quick-start setups |
|
| Generator, relaxed | true |
| Generator, recommended | true |
| Generator, strict | true |
The description is quoted from TypeScript's own option list (Apache-2.0); see the notices. The setup values are the checked configs on the quick-start pages. The generator rows are what the jsconfig generator writes at each level.
Examples
const user = { name: "Ada", role: "admin" };
let label = user.name;
label = label.toUppercase();error TS2551: Property 'toUppercase' does not exist on type 'string'. Did you mean 'toUpperCase'?
export { label };jsconfig.json for this example
{
"compilerOptions": {
"checkJs": true,
"noEmit": true,
"target": "es2022",
"module": "nodenext",
"types": []
},
"include": ["*.js"]
}// @ts-check
const names = ["Ada", "Grace", "Katherine"];
console.log(`${names.lenght} people`);error TS2551: Property 'lenght' does not exist on type 'string[]'. Did you mean 'length'?jsconfig.json for this example
{
"compilerOptions": {
"allowJs": true,
"checkJs": false,
"noEmit": true,
"target": "es2022",
"module": "nodenext",
"types": []
},
"include": ["*.js"]
}What it catches with no annotations
The first example has no JSDoc in it at all, and the checker still finds the typo. Without annotations, checkJs reports what it can work out from literals, the standard library, and the @types packages you install:
- misspelled properties and methods, often with a "Did you mean" suggestion
- calls with too many arguments (in JavaScript, a parameter without a JSDoc type counts as optional, so calls with fewer arguments are allowed)
- wrong types going into functions that do have types, such as the DOM and Node.js APIs
- imports of files and exports that do not exist
checkJs, allowJs and // @ts-check
allowJs lets JavaScript files into the program; checkJs decides whether their errors are reported. A config file named jsconfig.json already defaults allowJs to true, and in TypeScript 5.9 checkJs implies it as well, so most JavaScript projects only need checkJs.
You can also opt in one file at a time. A // @ts-check comment at the top of a file checks that file even when checkJs is off, which is how the second example works; // @ts-nocheck at the top of a file skips it when checkJs is on.
For a single line, // @ts-expect-error silences the error on the next line and is itself reported (TS2578) once there is nothing left to silence. // @ts-ignore silences the next line without that check, so it can hide a later, different error.
Switching it on in an existing codebase
In a large JavaScript codebase the first run can report hundreds of errors. A workable order: turn on checkJs without strict, fix or // @ts-nocheck the noisiest files, then turn on strict (or noImplicitAny first) once the count is manageable. The relaxed level of the jsconfig generator is that first step.
Gotchas
- Set
noEmittoo. A config file namedtsconfig.jsonwithallowJsand nooutDirmakestsctry to write output over your own.jsfiles, which fails with TS5055 ("Cannot write file … because it would overwrite input file"). A file namedjsconfig.jsondefaultsnoEmitto true. checkJschanges whattscreports and nothing else. Node.js and browsers run the same JavaScript whether it is on or off.- Without
checkJs(and without// @ts-check), editors still give completions and hovers in.jsfiles but show no type errors, which can look as if everything passes.
Questions
Do I need both allowJs and checkJs?
Not in TypeScript 5.9: checkJs turns on allowJs when you leave allowJs out, and a file named jsconfig.json defaults allowJs to true anyway. Setting both does no harm and makes the intent plain to the next reader.
Is checkJs the same as // @ts-check?
They turn on the same checks. checkJs applies to every JavaScript file in the project; // @ts-check applies to the one file it is written in, and works even when checkJs is off.
Sources
- TSConfig Reference: checkJs · TypeScript
- TSConfig Reference: allowJs · TypeScript
- TypeScript 2.3 release notes · TypeScript handbook
- Type Checking JavaScript Files · TypeScript handbook
- jsconfig.json · Visual Studio Code docs
Last reviewed 20 October 2026.