11

What does this initialization of a variable stand for:

var variable = variable  ||  {} ;

How and when should it be used?

skuntsel
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iuzzef
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  • See http://www.ecma-international.org/ecma-262/5.1/#sec-11.11. – Zeta Apr 05 '13 at 21:58
  • Thanks for including that link. It's nice to actually read this sentence, which has tripped me up in the past: "_The value produced by a && or || operator is not necessarily of type Boolean. The value produced will always be the value of one of the two operand expressions._" – jahroy Apr 05 '13 at 22:00
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    if this is in the top of a code module, it's for declaring a "namespace" object. See above mentioned question. – Alnitak Apr 05 '13 at 22:08
  • @skuntsel why did you add the `var`? It has the potential to completely change the semantics of the question. – Alnitak Apr 05 '13 at 22:23
  • @Alnitak I did that to stress out the initialization during declaration, and because I thought it was more appropriate to be used like that. Doesn't such an assignment (the one without `var`) produce an error if variable isn't declared? – skuntsel Apr 05 '13 at 22:38

4 Answers4

17

That line of code does the following:

IF variable is not defined (or has a falsey value) THEN set it to an empty object.

ELSE do nothing (technically speaking, variable gets assigned to itself)

In other words variable will be converted to an empty object if it is any of the following:

  • false
  • undefined
  • null
  • zero
  • NaN
  • an empty string

See toBoolean for the spec's definition of falsey values.

jahroy
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  • "Don't do anything" in terms of outcome, but internally it is an instruction that assigns the variable to itself (may be optimized by the compiler but is still an assignment expression), hence for performance Closure Compiler would compile it to `variable||(variable={})`. BTW you're missing `NaN` in your falsy values list. – Fabrício Matté Apr 05 '13 at 22:00
  • You forgot `NaN`. However, you should skip this list and reference to [`toBoolean`](http://www.ecma-international.org/ecma-262/5.1/#sec-9.2). See also http://www.ecma-international.org/ecma-262/5.1/#sec-11.11 – Zeta Apr 05 '13 at 22:01
  • Though "assigned to itself" is technically more accurate, I guess "don't do anything" is valid (in terms that it has no visible effect) and simpler to understand. Well, guess people can understand it either way. `:P` – Fabrício Matté Apr 05 '13 at 22:05
  • @FabrícioMatté "assigned to itself" isn't always a no-op - there might be a `setter` on the variable if it's a global (and as such a property of the `window` object). – Alnitak Apr 05 '13 at 22:17
  • @Alnitak Makes sense, I forgot about setters completely. Also, a getter could also perform some other ops before returning the value. – Fabrício Matté Apr 05 '13 at 22:20
1

If variable is undefined or false, it initializes it to an empty object.

Schleis
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1

It is to test if variable is initialized. If not, it initializes variable as an empty object. If it does exist, it does nothing, (technically assigns variable to itself).

RestingRobot
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1

If variable already exists use it, otherwise create an empty object

kevmc
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