TypedArray.prototype.findLastIndex()

findLastIndex() 方法以相反顺序迭代类型化数组，并返回满足所提供测试函数的第一个元素的索引。如果没有值满足测试函数，则返回 -1 。

另请参阅 findLast() 方法，该方法返回找到的元素的值，而不是其索引。

语法 

// Arrow function
findLastIndex((element) => { /* ... */ } )
findLastIndex((element, index) => { /* ... */ } )
findLastIndex((element, index, array) => { /* ... */ } )

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

// Inline callback function
findLastIndex(function(element) { /* ... */ })
findLastIndex(function(element, index) { /* ... */ })
findLastIndex(function(element, index, array){ /* ... */ })
findLastIndex(function(element, index, array) { /* ... */ }, thisArg)

Parameters

callbackFn

A function used to test elements in the typed array.

The function is called with the following arguments:

element

The current element in the typed array.

index

The index(position)of the current element in the typed array.

array

The typed array that findLastIndex() was called on.

The callback must return a truthy value to indicate an appropriate element has been found. The index of this element is returned by findLastIndex() .

thisArg Optional

Object to use as this when executing callbackFn .

Return value

The index of the last(highest-index)element in the array that passes the test. Otherwise -1 if no matching element is found.

Description

The findLastIndex() method executes the callbackFn function once for each index of the typed array in descending-index order until the callbackFn returns a truthy value. findLastIndex() then returns the index of that element and stops iterating through the typed array. If callbackFn never returns a truthy value, findLastIndex() returns undefined .

callbackFn is invoked with three arguments: the value of the element, the index of the element, and the typed array object being traversed.

If a thisArg parameter is provided to findLastIndex() , it will be used as the this for each invocation of the callbackFn . If it is not provided, then undefined is used.

The findLastIndex() method does not mutate the typed array on which it is called, but the function provided to callbackFn can.

The range of elements processed by findLastIndex() is set before the first invocation of callbackFn . Elements that are appended to the typed array after the call to findLastIndex() begins will not be visited by callbackFn . If an existing, unvisited element of the typed array is changed by callbackFn , its value passed to the visiting callbackFn will be the value at the time that findLastIndex() visits that element's index.

Warning: Concurrent modification of the kind described in the previous paragraph frequently leads to hard-to-understand code and is generally to be avoided(except in special cases).

实例

Find the index of a prime number in a typed array

The following example finds the index of the last element in the typed array that is a prime number(or returns -1 if there is no prime number). 

function isPrime(element) {
  if (element % 2 === 0 || element < 2) {
    return false;
  }
  for (let factor = 3; factor <= Math.sqrt(element); factor += 2) {
    if (element % factor === 0) {
      return false;
    }
  }
  return true;
}

let uint8 = new Uint8Array([4, 6, 8, 12]);
console.log(uint8.findLastIndex(isPrime));
// -1 (no primes in array)
uint8 = new Uint8Array([4, 5, 7, 8, 9, 11, 12]);
console.log(uint8.findLastIndex(isPrime));
// 11

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