no-allowjs-without-checkjs

Disallow using allowJs: true without also enabling checkJs: true.

Targeted pattern scope

The compilerOptions.allowJs and compilerOptions.checkJs fields in any tsconfig*.json file.

What this rule reports

This rule reports when compilerOptions.allowJs is set to true while compilerOptions.checkJs is either absent or explicitly false.

Why this rule exists

allowJs: true tells TypeScript to include .js (and .jsx) files in the compilation. This is commonly used when incrementally migrating a JavaScript project to TypeScript.

However, without checkJs: true, TypeScript includes those JavaScript files in output but does not type-check them. This means:

Type errors in .js files go unreported.
Incorrect argument types, undefined properties, and missing returns pass silently.
The TypeScript investment yields no safety benefit for the JavaScript portions of the codebase.

The intended pattern for mixed JS/TS projects is:

{
    "compilerOptions": {
        "allowJs": true,
        "checkJs": true
    }
}

This ensures all files — both .ts and .js — are subject to type analysis.

`jsconfig.json` consideration

In a jsconfig.json context (a JavaScript-only project), allowJs: true is implied and checkJs should always be set. This rule is included in the jsconfig preset for that reason.

❌ Incorrect

{
    "compilerOptions": {
        "allowJs": true
    }
}

JavaScript files are compiled but not type-checked.

{
    "compilerOptions": {
        "allowJs": true,
        "checkJs": false
    }
}

Type-checking is explicitly disabled for JavaScript files.

✅ Correct

{
    "compilerOptions": {
        "allowJs": true,
        "checkJs": true
    }
}

Both compilation and type-checking are enabled for JavaScript files.

{
    "compilerOptions": {}
}

Neither option is set — no JavaScript is included, so the rule does not apply.

When not to use it

Disable this rule when you intentionally include JavaScript files for compilation (e.g., copying .js build helpers into the output directory) without wanting type errors reported on those files. This is an uncommon but valid scenario, for example when using TypeScript as a build pipeline for legacy code.

Package documentation

Rule catalog ID: R026

Targeted pattern scope​

What this rule reports​

Why this rule exists​

jsconfig.json consideration​

❌ Incorrect​

✅ Correct​

When not to use it​

Package documentation​

Further reading​