[javascript] Ternary operators in JavaScript without an "else"

I've always had to put null in the else conditions that don't have anything. Is there a way around it?

For example,

condition ? x = true : null;

Basically, is there a way to do the following?

condition ? x = true;

Now it shows up as a syntax error.

FYI, here is some real example code:

!defaults.slideshowWidth ? defaults.slideshowWidth = obj.find('img').width()+'px' : null;

In your case i see the ternary operator as redundant. You could assign the variable directly to the expression, using ||, && operators.

!defaults.slideshowWidth ? defaults.slideshowWidth = obj.find('img').width()+'px' : null ;

will become :

defaults.slideshowWidth = defaults.slideshowWidth || obj.find('img').width()+'px';

It's more clear, it's more "javascript" style.

To use a ternary operator without else inside of an array or object declaration, you can use the ES6 spread operator, ...():

const cond = false;
const arr = [
  ...(cond ? ['a'] : []),
  'b',
];
    // ['b']

And for objects:

const cond = false;
const obj = {
  ...(cond ? {a: 1} : {}),
  b: 2,
};
    // {b: 2}

Original source

var x = condition || null;

No, it needs three operands. That's why they're called ternary operators.

However, for what you have as your example, you can do this:

if(condition) x = true;

Although it's safer to have braces if you need to add more than one statement in the future:

if(condition) { x = true; }

Edit: Now that you mention the actual code in which your question applies to:

if(!defaults.slideshowWidth)
    { defaults.slideshowWidth = obj.find('img').width()+'px'; }

You could write

x = condition ? true : x;

So that x is unmodified when the condition is false.

This then is equivalent to

if (condition) x = true

EDIT:

!defaults.slideshowWidth 
      ? defaults.slideshowWidth = obj.find('img').width()+'px' 
      : null 

There are a couple of alternatives - I'm not saying these are better/worse - merely alternatives

Passing in null as the third parameter works because the existing value is null. If you refactor and change the condition, then there is a danger that this is no longer true. Passing in the exising value as the 2nd choice in the ternary guards against this:

!defaults.slideshowWidth = 
      ? defaults.slideshowWidth = obj.find('img').width()+'px' 
      : defaults.slideshowwidth 

Safer, but perhaps not as nice to look at, and more typing. In practice, I'd probably write

defaults.slideshowWidth = defaults.slideshowWidth 
               || obj.find('img').width()+'px'

What about simply

    if (condition) { code if condition = true };

More often, people use logical operators to shorten the statement syntax:

!defaults.slideshowWidth &&
  (defaults.slideshowWidth = obj.find('img').width() + 'px');

But in your particular case the syntax can be even simpler:

defaults.slideshowWidth = defaults.slideshowWidth || obj.find('img').width() + 'px';

This code will return the defaults.slideshowWidth value if the defaults.slideshowWidth is evaluated to true and obj.find('img').width() + 'px' value otherwise.

See Short-Circuit Evaluation of logical operators for details.