indexOf()
ES5+Returns the first index at which a given element can be found in the array.
Syntax
array.indexOf(searchElement, fromIndex)Parameters
searchElement any Element to locate in the array
Return Value
The index of the element, or -1 if not found
Examples
const fruits = ['apple', 'banana', 'cherry'];
console.log(fruits.indexOf('banana'));
console.log(fruits.indexOf('mango'));📌 When to Use
Use indexOf() when you need the position of a primitive value in an array. It's useful for removing items, checking order, or when you need to work with the index value directly.
⚠️ Common Mistakes
Using if(indexOf(x)) instead of if(indexOf(x) !== -1) - index 0 is falsy!
indexOf() cannot find NaN - [NaN].indexOf(NaN) returns -1, use includes() or findIndex()
Expecting indexOf() to work with objects by value - it compares by reference
✅ Best Practices
Use includes() for simple existence checks when you don't need the index
Always compare with !== -1, not with > 0 (which misses index 0)
Use lastIndexOf() to find the last occurrence of a value
⚡ Performance Notes
indexOf() is O(n) and uses strict equality (===). For frequent lookups, convert the array to a Set or Map for O(1) access. The fromIndex parameter can improve performance by skipping already-searched portions.
🌍 Real World Example
Removing an Item from an Array
Find and remove a specific value from an array using indexOf and splice
const tags = ['javascript', 'typescript', 'react', 'vue'];
function removeTag(tagToRemove) {
const index = tags.indexOf(tagToRemove);
if (index !== -1) {
tags.splice(index, 1);
return true;
}
return false;
}
removeTag('vue');
console.log(tags); // ['javascript', 'typescript', 'react']