If I have an array such as:
let arr = ["So 2545", "Cool 1123", "Mos 1999"]
that is a subset of a larger array:
let largerArr = ["So 2545", "Fun 1023", "Loss 2009", "Cool 1123", "Mos 1999"]
How can I determine the index of each matching element in the larger array so the output would be another array (i.e. let output = [0,3,4])
You can use map() with indexOf():
const part = ["So 2545", "Cool 1123", "Mos 1999"];
const full = ["So 2545", "Fun 1023", "Loss 2009", "Cool 1123", "Mos 1999"];
const indexes = part.map(v => full.indexOf(v));
console.log(indexes);If you are dealing with large arrays, the overhead of building a lookup table (as suggested in other answers) might become worth it.
You could use .reduceRight() to make a look-up table of indexes, and then you .map() on your arr to map each element to an index from the look-up table:
const arr = ["So 2545", "Cool 1123", "Mos 1999"];
const largerArr = ["So 2545", "Fun 1023", "Loss 2009", "Cool 1123", "Mos 1999"];
const lut = largerArr.reduceRight((m, x, i) => m.set(x, i), new Map);
const output = arr.map(elem => lut.get(elem));
console.log(output);The purpose of using .reduceRight() is to find the first indexes that the match appears at in the array if the item appears more than once (not the last)