const query = succeed => (succeed? {value: 4} : undefined);

let value = 3;
for (let x of [true, false]) {
  try { value = query(x).value; } catch {} // 



The output is 4. If there is no .value field in the response, I want to keep the old value / don´t want the assignment to execute.
Notes:

• If value = query(x)?.value were used, it would assign undefined to value
• There is also the ??= operator, however it isn´t useful here, value ??= ... would only assign if value is currently null/undefined
• In CoffeeScript, value = query(x).value if query(x)?.value? achieves the desired behaviour without try/catch, although it's repetitive
• value = query(x)?.value ?? value works but isn't conditional assignment, the assignment still happens, if we were setting an Object's property the setter would be called unnecessarily. It is also repetitive
• This logic cannot be abstracted into a function i.e. value = smart(query(x), "value"), the assignment can´t be made conditional that way

Keywords?

null propagation, existence operator