Question mark and colon in JavaScript

Question

I came across the following line

hsb.s = max != 0 ? 255 * delta / max : 0;

What do the ? and : mean in this context?

Solution 1

It is called the Conditional Operator (which is a ternary operator).

It has the form of: condition ? value-if-true : value-if-false
Think of the ? as "then" and : as "else".

Your code is equivalent to

if (max != 0)
  hsb.s = 255 * delta / max;
else
  hsb.s = 0;

Solution 2

Properly parenthesized for clarity, it is

hsb.s = (max != 0) ? (255 * delta / max) : 0;

meaning return either

• 255*delta/max if max != 0
• 0 if max == 0

Solution 3

hsb.s = max != 0 ? 255 * delta / max : 0;

? is a ternary operator. It works like an if in conjunction with the :

!= means not equals

So, the long form of this line would be

if (max != 0) { //if max is not zero
  hsb.s = 255 * delta / max;
} else {
  hsb.s = 0;
}

Solution 4

This is probably a bit clearer when written with brackets as follows:

hsb.s = (max != 0) ? (255 * delta / max) : 0;

What it does is evaluate the part in the first brackets. If the result is true then the part after the ? and before the : is returned. If it is false, then what follows the : is returned.

Solution 5

?: is a short-hand condition for else {} and if(){} problems. So your code is interchangeable to this:

if(max != 0){
       hsb.s = 225 * delta / max
}
else {
       hsb.s = 0
}

MDN - Conditional (Ternary) Operator

View more solutions

Comments

[Jason S]

"?" isn't the ternary operator; "? :" is the ternary operator. Talking about "?" as the ternary operator is like talking about Abbott without Costello, Laurel without Hardy, Cheech without Chong....

[Greg]

Ok, ok... now I'm using an ambiguous pronoun, happy? :)

[Davy8]

To be pedantic, it's a ternary operator, which happens to be the only one in most programming languages. Any operator that works on 3 parts is a ternary operator, just like addition is a binary operator that operates on the preceding and following expressions (e.g. 1+2 the plus operates on 1 and 2), or negation is a unary operator (e.g. -x where the value of x is negated).

[Scott Lahteine]

That's one example of its use, but there's actually a shorter version of your statement, for those cases where you just want TRUE / FALSE: If 'expression' was just some variable with a number or string in it, "var x = !!expression" will make it into a boolean result.

[Mechanical snail]

@Davy8: And this one can be called the conditional-operator to be specific.

[jobmo]

"In JavaScript, a truthy value is a value that is considered true when evaluated in a Boolean context. All values are truthy unless they are defined as falsy (i.e., except for false, 0, "", null, undefined, and NaN)." This is why -1 is evaluated as true. (developer.mozilla.org/en-US/docs/Glossary/Truthy)

[Thanos Dodd]

What if you wanted to check two conditions?

[T.J. Crowder]

@ThanosDodd - Use nesting, or use if/else if/else instead.