如何找到从数组末尾开始的数组元素？

Question

JavaScript has array methods like indexOf and find for getting the first element in the array that matches the criteria. The counterpart to indexOf would be lastIndexOf , which searches starting from the end of the array instead. I'm wondering now if there is a counterpart to find that starts from the end of the array, like a native implementation of Ramda's findLast .

I would prefer to use neither array.slice().reverse().find() due to performance costs nor a for loop as it is verbose and not in the spirit of functional programming

Answer 1

No there is not, but you can polyfill it easily:

 Array.prototype.findLast = function(fn) {
  for(let i = this.length - 1; i >= 0; i--) 
    if(fn( this[i], i, this )) return this[i];
  return null;
 };



console.log([5,4,3,2,1].findLast(el => el > 3));

Answer 2

You can use reduceRight , which is in the spirit of functional programming. However, it's not as easy ( but possible ) to return early upon finding a match as it is in a for loop:

 const lastIndexOf = (needle, haystack) => haystack.reduceRight((a, e, i) => a >= 0 ? a : e === needle ? i : -1 , -1) ; const arr = [1,4,3,5,5,4,5]; console.log(lastIndexOf(4, arr)); console.log(lastIndexOf(2, arr)); 

There's also recursion, which has similar efficiency problems (stack frame overhead, no instantaneous early return, having to write a helper or extra conditionals, you'll blow the stack if the array is large...):

 const lastIndexOf = (needle, haystack, idx) => lastIndexOfHelper(needle, haystack, haystack.length - 1) ; const lastIndexOfHelper = (needle, haystack, idx) => idx < 0 || haystack[idx] === needle ? idx : lastIndexOfHelper(needle, haystack, idx - 1) ; const arr = [1,4,3,5,5,4,5]; console.log(lastIndexOf(4, arr)); console.log(lastIndexOf(2, arr)); 

Answer 3

Lodash has a _.findLastIndex() method which interates over a collection from right to left. https://lodash.com/docs/4.17.10#findLastIndex

Not sure as to its performance though