Using a JavaScript Map with array as key, why can't I get the stored value?

Question

My code initializes a Map object and uses arrays as key. When I try to use the map.get() method, I get \"undefined\" instead of the value I expect. What am I missing?

Answer 1

When you pass in a non-primitive to .get, you need to have used .set with a reference to the exact same object. For example, while setting, you do:

  theBoard.set([r, c], '-')

This creates an array [r, c] when that line runs. Then, in printBoardMap, your

  let mapKey = [r, c]

creates another array [r, c]. They're not the same array; if orig were the original array, mapKey !== orig.

You might consider setting and getting strings instead, for example '0_2' instead of [0, 2]:

theBoard.set(r + '_' + c, '-')

and

const mapKey = r + '_' + c;

(best to use const and not let when possible - only use let when you need to reassign the variable in question)