Finding the Index

Finding the index of the first element that passes a test: `findIndex()`

The findIndex() method in JavaScript searches through an array and returns the index of the first element that satisfies a provided testing function. If no such element is found, it returns -1. This method is useful for locating an element based on a specific condition without altering the original array.

The findIndex() function accepts a single argument: a callback function that tests each element in the array. This callback function can be customized to define the condition that the elements must meet to be considered.

//Syntax
findIndex(callbackFn(currentVal, index, arr))
findIndex(callbackFn(currentVal, index, arr), thisArg)

callbackFn(): A function to execute for each element in the array. It should return a truthy value to indicate a matching element has been found, and a falsy value otherwise. The function is called with the following arguments: currentVal, index and arr
currentVal: (Required) The current element being processed in the array.
index: (Optional) The index of the current element being processed in the array.
arr: (Optional) The array findIndex() was called upon.
thisArg: (Optional) The value to use as this when executing the callback function. By default it is undefined.

Examples

Example 1, using findIndex() to get the index of the first element that passes the test:

const ages = [3, 10, 18, 20];

let index = ages.findIndex(checkAge);

function checkAge(age) {
  return age > 18;
}

In this example:

A function checkAge() is passed as a callback to findIndex() function
findIndex() function will execute checkAge() callback function for each element of ages array
The condition age > 18 holds true for element value 20 at index 3 in ages array, therefore findIndex() will return 3

Example 2, findIndex() returns -1 when no element matches the callback function condition:

const ages = [3, 10, 13, 15];

let index = ages.findIndex(checkAge);

function checkAge(age) {
  return age > 18;
}

In this case, no element of ages satisfies the age > 18 condition so the findIndex() function will return -1.

Notes

Executes a function on each array element
Returns the index (position) of the first element that passes the test
Returns -1 if no match is found
Does not executes the function for empty array elements
Does not change the original array

Finding the Index