How to use rest and spread operators with functions

Short answer

Rest parameters (...args) gather remaining arguments into an array, while the spread operator (...arr) expands an array into individual arguments or elements.

Steps

1. Use rest parameters in function definitions: javascript function sum(...numbers) { return numbers.reduce((a, b) => a + b, 0); } console.log(sum(1, 2, 3)); // 6
2. Combine rest with named parameters: javascript function log(level, ...messages) { console[level](...messages); }
3. Use spread to call functions with array elements: javascript const nums = [10, 20, 30]; console.log(Math.max(...nums)); // 30
4. Use spread to copy and merge arrays: javascript const a = [1, 2]; const b = [3, 4]; const merged = [...a, ...b];

Tips

• Rest must be the last parameter in a function signature.
• Spread works on any iterable, including strings and NodeLists.

Common issues

• Confusing rest and spread syntax context; rest is in declarations, spread in calls/literals.
• Shallow copying with spread means nested objects still share references.