Proof that JavaScript maps index functions by identity, not source string.

map-func-identity.js

const funcString = '() => "foo"';
const funcA = new Function(funcString);
const funcB = new Function(funcString);

const map = new Map();
map.set(funcA, 'a');
map.set(funcB, 'b');
const aVal = map.get(funcA);
console.log(aVal === 'a');