no-deprecated-target

Disallow target values of "ES3" or "ES5" in compilerOptions.

Targeted pattern scope

The compilerOptions.target field in any tsconfig*.json file.

What this rule reports

This rule reports when compilerOptions.target is set to "ES3" or "ES5".

Why this rule exists

TypeScript's target option controls which JavaScript version the compiler emits. Targeting an old version causes TypeScript to down-level modern syntax features — arrow functions become function expressions, class becomes prototype chains, template literals become string concatenation, and so on.

As of 2024, ES2015 (ES6) support is universal across all actively maintained runtimes:

Runtime	ES2015 support since
Node.js	v6.0 (April 2016)
Chrome / V8	v51 (May 2016)
Firefox	v54 (June 2017)
Safari / JavaScriptCore	v10 (September 2016)
Edge (Chromium)	v14 (August 2016)

Consequences of targeting ES3 or ES5:

Larger output: Down-leveled syntax is verbose. Arrow functions, classes, and destructuring all expand into more code.
Performance regressions: Some engines have optimised native ES2015+ syntax paths that down-leveled equivalents do not benefit from.
False security: The project may still use npm packages that ship ES2015+ code, meaning the bundle is not actually ES5-safe end-to-end.
Decorator interop issues: The legacy decorator implementation in TS 5.x behaves differently under ES5 vs ES2022 targets.

If you need to support old runtimes (pre-ES2015), use a separate transpilation step (e.g., Babel) targeting those runtimes after TypeScript has done its type checking against a modern target. This gives you accurate type checking and the correct output.

❌ Incorrect

{
    "compilerOptions": {
        "target": "ES3"
    }
}

Targets a 1999-era JavaScript specification.

{
    "compilerOptions": {
        "target": "ES5"
    }
}

Targets IE-era JavaScript; no modern runtime requires this.

✅ Correct

{
    "compilerOptions": {
        "target": "ES2022"
    }
}

Targets a version all modern runtimes support natively.

{
    "compilerOptions": {
        "target": "ESNext"
    }
}

Targets the latest ECMAScript version — appropriate for Vite/bundler projects where a separate minifier handles compatibility.

When not to use it

Disable this rule when the project genuinely targets an environment that only supports ES3 or ES5 — such as legacy IoT firmware, embedded browsers from before 2016, or environments with a strict regulatory mandate to support Internet Explorer. In those cases, the old target is intentional and the trade-offs are accepted.

Package documentation

Rule catalog ID: R027

Targeted pattern scope​

What this rule reports​

Why this rule exists​

❌ Incorrect​

✅ Correct​

When not to use it​

Package documentation​

Further reading​