const targetNumIndex = prevNumstargetNum

This line of code assumes that there is an array or object called "prevNums" that stores some previous numbers and their indices. It also assumes that there is a variable called "targetNum" that contains the number we are trying to find the index of.

The purpose of this line is to retrieve the index of the "targetNum" in the "prevNums" array or object. It does this by accessing the value stored in the "prevNums" object at the key corresponding to the "targetNum" value. In other words, if "prevNums" is an object with keys representing numbers and values representing their indices, this line retrieves the index of the "targetNum" by looking it up in the "prevNums" object.

If the "targetNum" is not found in the "prevNums" object, the value of "targetNumIndex" will be undefined.