JavaScript - find last index of element inside array by certain condition

You can use findIndex() method on reversed array.

The findIndex() method returns the index of the first element in an array that satisfies the provided testing function. Used on reversed array will give you the last index.

Note:

Use slice() to copy the original array because reverse() method will mutate it.

Practical example

// ONLINE-RUNNER:browser;

function findLastIndex(array, key, value) {
    var index = array
        .slice()
        .reverse()
        .findIndex(function(x) {
            return x[key] === value;
        });

    var count = array.length - 1;
    var finalIndex = index >= 0 ? count - index : index;
    return finalIndex;
}


// Usage example:

var objects = [
    { id: 1, type: 'A' },
    { id: 2, type: 'B' },
    { id: 3, type: 'A' },
    { id: 4, type: 'B' },
    { id: 5, type: 'A' },
];

console.log(findLastIndex(objects, 'type', 'B')); // 3
console.log(findLastIndex(objects, 'type', 'C')); // -1

References

1. Array.prototype.slice() - JavaScript | MDN
2. Array.prototype.reverse() - JavaScript | MDN
3. Array.prototype.findIndex() - JavaScript | MDN