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

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.

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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.

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;
}

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

This is a one-line shorthand for an if-else statement. It's called the conditional operator.¹

Here is an example of code that could be shortened with the conditional operator:

var userType;
if (userIsYoungerThan18) {
  userType = "Minor";
} else {
  userType = "Adult";
}

if (userIsYoungerThan21) {
  serveDrink("Grape Juice");
} else {
  serveDrink("Wine");
}

This can be shortened with the ?: like so:

var userType = userIsYoungerThan18 ? "Minor" : "Adult";

serveDrink(userIsYoungerThan21 ? "Grape Juice" : "Wine");

Like all expressions, the conditional operator can also be used as a standalone statement with side-effects, though this is unusual outside of minification:

userIsYoungerThan21 ? serveGrapeJuice() : serveWine();

They can even be chained:

serveDrink(userIsYoungerThan4 ? 'Milk' : userIsYoungerThan21 ? 'Grape Juice' : 'Wine');

Be careful, though, or you will end up with convoluted code like this:

var k = a ? (b ? (c ? d : e) : (d ? e : f)) : f ? (g ? h : i) : j;

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¹Often called "the ternary operator," but in fact it's just a ternary operator [an operator accepting three operands]. It's the only one JavaScript currently has, though.