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JavaScript

Ben Os
Ben Os
20,008 Points
Posted by Ben Os
Ben Os
20,008 Points

? x : y; --- What is this syntax in Js?

I have seen the following code:

if (obj.a === undefined) { a = obj.b === undefined ? b : obj.b;

I know that a and b are properties of obj and I understand that the code says the following (hope I understand this correct):

If obj.a is undefined, put obj.b in it, if the value of property b is obj.b

Is my understanding of the question-mark and colon syntax accurate in this context?

Dave McFarland

3 Answers

andren
andren
28,558 Points
andren
andren
28,558 Points

The ? : syntax is an example of a ternary operator.

The structure of a ternary operator is like this: (boolean) ? returnedIfTrue : returnedIfFalse.

It is basically a short way to perform a one line if/else statement.

So a = obj.b === undefined ? b : obj.b; is actually saying if obj.b === undefined is true, then set a equal to b. If it is false then set a equal to obj.b.

It is the same as this if/else statement:

if (obj.b === undefined) {
    a = b;
} else {
    a = obj.b;
}

It is just a shortened version.

Dave McFarland
STAFF
Dave McFarland
Treehouse Teacher
Dave McFarland
Dave McFarland
Treehouse Teacher

Hi Ben Aharoni

As Andren and Aaron pointed out this is called a ternary operator. We have a short workshop you could watch called Exploring JavaScript Conditionals which includes a video about the Ternary Operator