How to conditionally add a member to an object?

I would like to create an object with a member added conditionally.
The simple approach is:

var a = {};
if (someCondition)
    a.b = 5;

Now, I would like to write a more idiomatic code. I am trying:

a = {
    b: (someCondition? 5 : undefined)
};

But now, b is a member of a whose value is undefined. This is not the desired result.

Is there a handy solution?

Update

I seek for a solution that could handle the general case with several members.

a = {
  b: (conditionB? 5 : undefined),
  c: (conditionC? 5 : undefined),
  d: (conditionD? 5 : undefined),
  e: (conditionE? 5 : undefined),
  f: (conditionF? 5 : undefined),
  g: (conditionG? 5 : undefined),
 };

In pure Javascript, I cannot think of anything more idiomatic than your first code snippet.

If, however, using the jQuery library is not out of the question, then $.extend() should meet your requirements because, as the documentation says:


Undefined properties are not copied.

Therefore, you can write:

var a = $.extend({}, {
    b: conditionB ? 5 : undefined,
    c: conditionC ? 5 : undefined,
    // and so on...
});

And obtain the results you expect (if conditionB is false, then b will not exist in a).

Stack Overflow Q#11704267, score: 142