var obj = { a:1 }

​

function setNumber(key, value) {

  obj[key] = value;

}

​

function setCall(key, getValue) {

  obj[key] = getValue(obj);

}

​

function setTypecheck(key, value) {

  obj[key] = typeof value === "number" ? value : value(obj);

}

​

function getValue() {

  return 100;

}