Why I get "Invalid left-hand side in assignment"?

Question

There is code:

function search(list, q){
  var result = {};
  for(let id in list)(
    (!q.id    || (id == q.id)) &&
    (!q.name  || (list[id].name.search(q.name) > -1)) &&
    result[id] = list[id]
  );

  return result;
}

I get this error:

Uncaught ReferenceError: Invalid left-hand side in assignment    script.js:4

Why "&&" is wrong?

Answer 1

The problem is that the assignment operator, =, is a low-precedence operator, so it's being interpreted in a way you don't expect. If you put that last expression in parentheses, it works:

  for(let id in list)(
    (!q.id    || (id == q.id)) &&
    (!q.name  || (list[id].name.search(q.name) > -1)) &&
    (result[id] = list[id])
  );