How do you use the ? : (conditional) operator in JavaScript?

Question

In simple words, what is the ?: (conditional, "ternary") operator and how can I use it?

Answer 1

It's called the ternary operator. For some more info, here's another question I answered regarding this:

How to write an IF else statement without 'else'

Answer 2

I want to add some to the given answers.

In case you encounter (or want to use) a ternary in a situation like 'display a variable if it's set, else...', you can make it even shorter, without a ternary.

---

Instead of:

var welcomeMessage  = 'Hello ' + (username ? username : 'guest');

You can use:

var welcomeMessage  = 'Hello ' + (username || 'guest');

This is Javascripts equivallent of PHP's shorthand ternary operator ?:

Or even:

var welcomeMessage  = 'Hello ' + (username || something || maybethis || 'guest');

It evaluates the variable, and if it's false or unset, it goes on to the next.

Answer 3

It's an if statement all on one line.

So

var x=1;
(x == 1) ? y="true" : y="false";
alert(y);

The expression to be evaluated is in the ( )

If it matches true, execute the code after the ?

If it matches false, execute the code after the :

Answer 4

It's a little hard to google when all you have are symbols ;) The terms to use are "JavaScript conditional operator".

If you see any more funny symbols in JavaScript, you should try looking up JavaScript's operators first: Mozilla Developer Center's list of operators. The one exception you're likely to encounter is the $ symbol.

To answer your question, conditional operators replace simple if statements. An example is best:

var insurancePremium = age > 21 ? 100 : 200;

Instead of:

var insurancePremium;

if (age > 21) {
    insurancePremium = 100;
} else {
    insurancePremium = 200;
}

Answer 5

This is probably not exactly the most elegant way to do this. But for someone who is not familiar with ternary operators, this could prove useful. My personal preference is to do 1-liner fallbacks instead of condition-blocks.

  // var firstName = 'John'; // Undefined
  var lastName = 'Doe';

  // if lastName or firstName is undefined, false, null or empty => fallback to empty string
  lastName = lastName || '';
  firstName = firstName || '';

  var displayName = '';

  // if lastName (or firstName) is undefined, false, null or empty
  // displayName equals 'John' OR 'Doe'

  // if lastName and firstName are not empty
  // a space is inserted between the names
  displayName = (!lastName || !firstName) ? firstName + lastName : firstName + ' ' + lastName;


  // if display name is undefined, false, null or empty => fallback to 'Unnamed'
  displayName = displayName || 'Unnamed';

  console.log(displayName);

Ternary Operator

Answer 6

It's called the 'ternary' or 'conditional' operator.

Example

The ?: operator can be used as a shortcut for an if...else statement. It is typically used as part of a larger expression where an if...else statement would be awkward. For example:

var now = new Date();
var greeting = "Good" + ((now.getHours() > 17) ? " evening." : " day.");

The example creates a string containing "Good evening." if it is after 6pm. The equivalent code using an if...else statement would look as follows:

var now = new Date();
var greeting = "Good";
if (now.getHours() > 17)
   greeting += " evening.";
else
   greeting += " day.";

From MSDN JS documentation.

Basically it's a shorthand conditional statement.

Also see:

• Operator precedence with Javascript Ternary operator
• Wikipedia