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

Question

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),  }; 

Answer 1

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}) }