In JavaScript, how to conditionally add a member to an object?

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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),
 };

24 s
24

I think @InspiredJW did it with ES5, and as @trincot pointed out, using es6 is a better approach. But we can add a bit more sugar, by using the spread operator, and logical AND short circuit evaluation:

const a = {
   ...(someCondition && {b: 5})
}

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