Find last matching object in array of objects

Question

I have an array of objects. I need to get the object type (\"shape\" in this example) of the last object, remove it, and then find the index of the previous object in the ar

Answer 1

You should use filter! filter takes a function as an argument, and returns a new array.

var roundFruits = fruits.filter(function(d) {
 // d is each element of the original array
 return d.shape == "round";
});

Now roundFruits will contain the elements of the original array for which the function returns true. Now if you want to know the original array indexes, never fear - you can use the function map. map also operates on an array, and takes a function which acts on the array. we can chain map and filter together as follows

var roundFruits = fruits.map(function(d, i) {
  // d is each element, i is the index
  d.i = i;  // create index variable
  return d;
}).filter(function(d) {
  return d.shape == "round"
});

The resulting array will contain all objects in the original fruits array for which the shape is round, and their original index in the fruits array.

roundFruits = [
{ 
    shape: round,
    name: orange,
    i: 0
},
{ 
    shape: round,
    name: apple,
    i: 1
},
{ 
    shape: round,
    name: grapefruit
    i: 4
}
]

Now you can do whatever you need to with the exact knowledge of the location of the relevant data.

// get last round element
fruits[4];

Answer 2

var fruit = fruits.slice().reverse().find(fruit => fruit.shape === currentShape);

Answer 3

I would suggest another nice solution which doesn't bother cloning a new object using reverse().

I use reduceRight to does the job instead.

function findLastIndex(array, fn) {
  if (!array) return -1;
  if (!fn || typeof fn !== "function") throw `${fn} is not a function`;
  return array.reduceRight((prev, currentValue, currentIndex) => {
    if (prev > -1) return prev;
    if (fn(currentValue, currentIndex)) return currentIndex;
    return -1;
  }, -1);
}

And usage

findLastIndex([1,2,3,4,5,6,7,5,4,2,1], (current, index) => current === 2); // return 9

findLastIndex([{id: 1},{id: 2},{id: 1}], (current, index) => current.id === 1); //return 2

Answer 4

You can transform your array to an array boolean type and get the last true index.

const lastIndex = fruits.map(fruit => 
  fruit.shape === currentShape).lastIndexOf(true);

Answer 5

plain JS:

var len = fruits.length, prev = false;
while(!prev && len--){
    (fruits[len].shape == currentShape) && (prev = fruits[len]);
}

lodash:

_.findLast(fruits, 'shape', currentShape);

Answer 6

This is a solution that does not depend on reverse, and therefore does not require "cloning" the original collection.

const lastShapeIndex = fruits.reduce((acc, fruit, index) => (
    fruit.shape === currentShape ? index : acc
), -1);