JavaScript includes() vs indexOf() - Element Existence Checks

includes() vs indexOf()

Both methods check whether an array contains a specific value, but they differ in return type and NaN handling.

Array.includes()

Returns a boolean — simple and clear:

const fruits = ["apple", "banana", "cherry"];
fruits.includes("banana");  // true
fruits.includes("grape");   // false

Array.indexOf()

Returns the index of the first occurrence, or -1 if not found:

const fruits = ["apple", "banana", "cherry"];
fruits.indexOf("banana");  // 1
fruits.indexOf("grape");   // -1

Key Difference: NaN Handling

const arr = [1, 2, NaN, 4];

arr.includes(NaN);  // true  ✓
arr.indexOf(NaN);   // -1    ✗ (cannot find NaN!)

includes() uses the SameValueZero algorithm, which correctly identifies NaN. indexOf() uses strict equality (===), and since NaN !== NaN, it cannot find NaN values.

fromIndex Parameter

Both support a second argument to start searching from a specific index:

const arr = ["a", "b", "c", "a", "b"];
arr.includes("a", 2);  // true (finds the second "a")
arr.indexOf("a", 2);   // 3 (index of second "a")

When to Use Which

Scenario	Use
Just need true/false	includes()
Need the position	indexOf()
Array may contain NaN	includes()
Need custom condition	find() or findIndex()

Performance: When to Use Set

For frequent lookups on large arrays, convert to a Set first:

// O(n) per lookup
array.includes(value);

// O(1) per lookup after O(n) setup
const set = new Set(array);
set.has(value);

If you are checking membership in a loop, a Set is significantly faster for arrays larger than ~100 elements.