I have this code in JavaScript:
var JSON;
JSON||(JSON={});
Can you tell what this code is doing?
6 Answers
var JSON is declaring a global (or other scope) variable.
JSON||(JSON={}); is first checking to see if JSON evaluates to true or false, and if false, then JSON is set to an empty object;
It is pretty pointless in that context,
var JSON;
creates a variable named JSON
The second part,
JSON||(JSON={});
is equivalent to
if(!JSON){
JSON = {};
}
This can all be skipped and written as
var JSON = {};
3 Comments
Quentin
If you skip the test, then you will always overwrite whatever it contains. That's usually Not A Good Thing in any case where someone has bothered to check if the variable has a truthy value.
Andrew
@Quentin that only applies if there is more code before the set, which there is not.
Quentin
— I think that assuming the code in the question represents the complete program is probably unsafe.
It scopes a variable called JSON, then uses the shortcircuit nature of the || operator to assign an empty object to said variable unless that variable has a truthy value.
4 Comments
Daniel A. White
@Robotsushi - thats what its called. theres also falsey.
Ry-
@Robotsushi: Isn't that the official term?
BentOnCoding
I just don't hear those expression used often enough =D
Ry-
@DanielA.White: I thought it was "falsy." :)
Its just making JSON an empty object.
Comments
This code is doing the following:
- Declare variable
JSON(JSON === undefined) - Assign an empty object (
{}) toJSONifJSONis a falsey value.
Falsey values include: null, undefined, "" (empty string), 0, NaN, false.
JSON has been declared via var JSON, so is set to undefined, meaning that the right hand-side of the operation JSON||... will be executed. In other words, all the code is achieving is:
var JSON = {};
4 Comments
Quentin
It scopes the variable. It might have a value from somewhere else.
James
@Quentin, Exactly -- the value must be
undefined because the variable is declared via var JSON. If it had another value prior to that declaration it won't matter since JSON||.. runs after that.Quentin
— No,
var gets hosted and doesn't reset a variable with a value defined earlier in the scope. I've seen: function (foo) { var foo … } a number of times. The var is redundant because arguments are locally scoped automatically.James
Ah I see. I didn't think of the case where the var would have already been defined in the same scope.
I think this says : if the var 'JSON' is null create a blank javascript obect.
1 Comment
Quentin
No. It tests if it is
null or undefined or any other falsy value. and if it is it assigns a JavaScript object. There is no JSON there (except as the name of a poorly named variable).
var JSON = {};, which creates a new object and assigns it to the variableJSON.