To find last index of element inside array by certain condition with JavaScript, we use the map and lastIndexOf methods.

For instance, we write

const lastIndex = elements.map((e) => e.a).lastIndexOf("something");

to call map with a callback that returns property a‘s value in each object e in elements.

Then we call lastIndexOf to get the last index of the a value that equals 'something' in the returned array.