includes()
ES7+Determines whether an array includes a certain value among its entries.
Syntax
array.includes(searchElement, fromIndex)Parameters
searchElement any The value to search for
fromIndex number optionalPosition to start searching from
Return Value
true if the value is found, otherwise false
Examples
const fruits = ['apple', 'banana', 'cherry'];
console.log(fruits.includes('banana'));
console.log(fruits.includes('mango'));📌 When to Use
Use includes() for simple membership tests with primitive values. It's the clearest way to check if an array contains a specific value, especially when you don't need the index or the element itself.
⚠️ Common Mistakes
Using includes() with objects - it uses strict equality (===), so object references must match exactly
Forgetting that includes() can find NaN (unlike indexOf())
Using indexOf() !== -1 when includes() would be more readable
✅ Best Practices
Use includes() for cleaner code: arr.includes(val) instead of arr.indexOf(val) !== -1
For object lookups, use some() with a custom comparison function instead
Combine with Set for frequent lookups: new Set(arr).has(val) is O(1)
⚡ Performance Notes
includes() is O(n) for arrays - it checks each element until it finds a match. For frequent membership tests on large arrays, convert to a Set first for O(1) lookup time. Unlike indexOf(), includes() correctly identifies NaN.
🌍 Real World Example
Checking User Roles for Access Control
Verify if a user has a specific role before showing admin features
const userRoles = ['user', 'editor'];
const adminRoles = ['admin', 'superadmin'];
const isAdmin = adminRoles.some(role => userRoles.includes(role));
// isAdmin: false
// Simple feature flag check
const enabledFeatures = ['dark-mode', 'notifications', 'analytics'];
if (enabledFeatures.includes('dark-mode')) {
enableDarkMode();
}