Common Code Quality Issues

Common pitfalls

Unused variables (no-unused-vars), undefined variables (no-undef), unnecessary variables (no-var), and missing semicolons are the top issues ESLint catches.

Before and after

Code with issues (left) vs. ESLint-compliant code (right).

// BEFORE (issues)
var x = 5;
let result = x + y; // y undefined, x unused
console.log(result)

// AFTER (fixed)
const result = 5 + 10;
console.log(result);

Output:

Fixed: removed unused var x, defined y, added semicolon

Auto-fix when possible

Run eslint --fix to automatically correct many issues. But review changes—some fixes require manual intervention.

Quick check

Quick check: What does 'no-undef' rule catch?

• Variables used without being defined.
• Variables that are never used.
• Functions without return statements.