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 executingcallbackFn
.
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