JavaScript trying to add key/value pair with Map.property.set()

script.js

document.querySelector('#outcome5').innerHTML = inventors.sort((a, b) => {
  const lastPersonAge = a.passed - a.year;
  const nextPersonAge = b.passed - b.year;
  a.set(age, lastPersonAge);
  b.set(age, nextPersonAge);
  return nextPersonAge > lastPersonAge ? 1 : -1;
}).map(inventor => ` ${inventor.first} ${inventor.last} — age: ${inventor.passed - inventor.year}`);