TypedArray.prototype.findLast()

findLast()方法以相反的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值。如果没有值满足测试函数，则返回undefined。

另请参阅findLastIndex()方法，该方法返回找到的元素的索引，而不是其值。

语法

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

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

// Inline callback function
findLast(function(element) { /* ... */ })
findLast(function(element, index) { /* ... */ })
findLast(function(element, index, array){ /* ... */ })
findLast(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 findLast() was called on.

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

thisArg Optional

Object to use as this when executing callbackFn.

Return value

The element in the typed array with the highest index value that satisfies the provided testing function;undefined if no matching value is found.

Description

The findLast() method executes the callbackFn function once for each index of the typed array in descending-index order until the callbackFn returns a truthy value.findLast() then returns the value of that element and stops iterating through the typed array. If callbackFn never returns a truthy value,findLast() 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 findLast(), it will be used as the this for each invocation of the callbackFn. If it is not provided, then undefined is used.

The findLast() 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 findLast() is set before the first invocation of callbackFn. Elements that are appended to the typed array after the call to findLast() 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 findLast() 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).

Examples

Find a prime number in a typed array

The following example finds the value of the last element in the typed array that is a prime number(or returns undefined 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.findLast(isPrime)); // undefined (no primes in array)
uint8 = new Uint8Array([4, 5, 7, 8, 9, 11, 12]);
console.log(uint8.findLast(isPrime)); // 11

All elements are visited and may be modified by the callback

The following examples show that all elements are visited, and that the value passed to the callback is their value when visited:

// Declare array with no elements at indexes 2, 3, and 4
// The missing elements will be initialized to zero.
const uint8 = new Uint8Array([0, 1, , , , 5, 6]);

// Iterate through the elements in reverse order.
// Note that all elements are visited.
uint8.findLast((value, index) => {
  console.log(`Visited index ${index} with value ${value}`);
});

// Shows all indexes, including deleted
uint8.findLast((value, index) => {
  // Modify element 3 on first iteration
  if (index === 6) {
    console.log("Set uint8[3] to 44");
    uint8[3] = 44;
  }
  // Element 3 is still visited but will have a new value.
  console.log(`Visited index ${index} with value ${value}`);
});
// Visited index 6 with value 6
// Visited index 5 with value 5
// Visited index 4 with value 0
// Visited index 3 with value 0
// Visited index 2 with value 0
// Visited index 1 with value 1
// Visited index 0 with value 0
// Deleting uint8[5] with value 5
// Visited index 6 with value 6
// Visited index 5 with value 5
// Visited index 4 with value 0
// Visited index 3 with value 44
// Visited index 2 with value 0
// Visited index 1 with value 1
// Visited index 0 with value 0