Javascript equivalent of Python’s dict.setdefault?

if (!('key' in d)) d.key = value;

or

'key' in d || (d.key = value);

(The last one uses the short-circuit behavior of conditional expressions.)

“d.key || (d.key = value)” suggested in the other answer is buggy: if an element exists but evaluates to false (false, null, 0, -0, "", undefined, NaN), it will be silently overwritten.

• While this might be intended, it’s most probably not intended for all of the aforementioned cases. So in such a case, check for the values separately:

  if (!('key' in d) || d.key === null) d.key = value;