Array.prototype.findIndex

The findIndex() method returns the index of the first element in an array that satisfies the provided testing function. If no elements satisfy the testing function, -1 is returned.

See also the find() method, which returns the first element that satisfies the testing function (rather than its index).

Syntax

// Arrow function
findIndex((element) => { /* … */ })
findIndex((element, index) => { /* … */ })
findIndex((element, index, array) => { /* … */ })

// Callback function
findIndex(callbackFn)
findIndex(callbackFn, thisArg)

// Inline callback function
findIndex(function (element) { /* … */ })
findIndex(function (element, index) { /* … */ })
findIndex(function (element, index, array) { /* … */ })
findIndex(function (element, index, array) { /* … */ }, thisArg)

Parameters

• 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.

    The function is called with the following arguments:

    • element
      • : The current element being processed in the array.
    • index
      • : The index of the current element being processed in the array.
    • array
      • : The array findIndex() was called upon.

• thisArg optional

  • : A value to use as this when executing callbackFn.

Return value

The index of the first element in the array that passes the test. Otherwise, -1.

Description

The findIndex() is an iterative method. It calls a provided callbackFn function once for each element in an array in ascending-index order, until callbackFn returns a truthy value. findIndex() then returns the index of that element and stops iterating through the array. If callbackFn never returns a truthy value, findIndex() returns -1.

callbackFn is invoked for every index of the array, not just those with assigned values. Empty slots in sparse arrays behave the same as undefined.

findIndex() does not mutate the array on which it is called, but the function provided as callbackFn can. Note, however, that the length of the array is saved before the first invocation of callbackFn. Therefore:

• callbackFn will not visit any elements added beyond the array's initial length when the call to findIndex() began.
• Changes to already-visited indexes do not cause callbackFn to be invoked on them again.
• If an existing, yet-unvisited element of the array is changed by callbackFn, its value passed to the callbackFn will be the value at the time that element gets visited. Deleted elements are visited as if they were undefined.

The findIndex() method is generic. It only expects the this value to have a length property and integer-keyed properties.