How to Use findIndex in JavaScript: Syntax and Examples

Use the findIndex method on an array to get the index of the first element that satisfies a condition defined by a callback function. It returns the index if found, or -1 if no element matches.

Syntax

The findIndex method is called on an array and takes a callback function as its argument. This callback receives each element, its index, and the whole array, and should return true for the element you want to find.

• callback(element, index, array): Function to test each element.
• Returns: The index of the first element that passes the test, or -1 if none do.

javascript

array.findIndex((element, index, array) => {
  // return true when condition is met
});

Example

This example finds the index of the first number greater than 10 in an array.

javascript

const numbers = [4, 9, 16, 25];
const index = numbers.findIndex(num => num > 10);
console.log(index);

Output

2

Common Pitfalls

One common mistake is forgetting that findIndex returns -1 if no element matches, which can cause bugs if not checked. Also, the callback must return a boolean; returning a non-boolean can lead to unexpected results.

Another pitfall is using findIndex on an empty array or without a proper callback, which will always return -1.

javascript

const arr = [1, 2, 3];
// Wrong: callback returns non-boolean values
const wrongIndex = arr.findIndex(num => num > 5 ? 'yes' : 'no');
console.log(wrongIndex); // Output: -1 because 'yes' and 'no' are truthy but not boolean

// Right: callback returns boolean
const rightIndex = arr.findIndex(num => num > 5);
console.log(rightIndex); // Output: -1

Output

-1
-1

Quick Reference

Method	Description	Return Value
findIndex(callback)	Finds index of first element matching condition	Index (number) or -1 if not found
callback(element)	Function to test each element	Boolean (true to match)
Returns -1	If no element matches the condition	Number -1

Key Takeaways

Use findIndex to get the index of the first array element that meets a condition.

The callback function must return a boolean to indicate a match.

If no element matches, findIndex returns -1, so always check for this.

findIndex does not change the original array.

It is useful for finding positions without looping manually.