no-unused-expressions

Disallow unused expressions.

✅

Extending "plugin:@typescript-eslint/recommended" in an ESLint configuration enables this rule.

This rule extends the base eslint/no-unused-expressions rule. It supports TypeScript-specific expressions:

• Marks directives in modules declarations ("use strict", etc.) as not unused
• Marks the following expressions as unused if their wrapped value expressions are unused:
  • Assertion expressions: x as number;, x!;, <number>x;
  • Instantiation expressions: Set<number>;

Although the type expressions never have runtime side effects (that is, x!; is the same as x;), they can be used to assert types for testing purposes.

Examples

❌ Incorrect

Set<number>;
1 as number;
window!;

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✅ Correct

function getSet() {
  return Set;
}

// Funtion calls are allowed, so type expressions that wrap function calls are allowed
getSet()<number>;
getSet() as Set<unknown>;
getSet()!;

// Namespaces can have directives
namespace A {
  'use strict';
}

Open in Playground

Options

See eslint/no-unused-expressions's options.

How to Use

Flat Config

eslint.config.mjs

export default tseslint.config({
  rules: {
    // Note: you must disable the base rule as it can report incorrect errors
    "no-unused-expressions": "off",
    "@typescript-eslint/no-unused-expressions": "error"
  }
});

Legacy Config

.eslintrc.cjs

module.exports = {
  "rules": {
    // Note: you must disable the base rule as it can report incorrect errors
    "no-unused-expressions": "off",
    "@typescript-eslint/no-unused-expressions": "error"
  }
};

Try this rule in the playground ↗

Resources

• Rule source
• Test source

^{Taken with ❤️ from ESLint core.}