What is this in [removed] “var var1 = var1 || []”

Question

I just want to increase my core javascript knowledge.

Sometimes I see this statement but I don\'t know what it does:

var var1 = var1 || [];

Answer 1

While this has been pointed out as 'being a silly statement', I present the following two counters:

(Just to keep people on their toes and reinforce some of the "finer details" of JavaScript.)

1)

var is the variable is already local. E.g.

function x (y) {
  var y = y || 42 // redeclaration warning in FF, however it's "valid"
  return y
}
x(true) // true
x()     // 42

2)

var is function-wide annotation (it is "hoisted" to the top) and not a declaration at the point of use.

function x () {
  y = true
  var y = y || 42
}
x()      // true

I do not like code like either of the preceding, but...

Because of the hoisting, and allowed re-declarations, the code in the post has these semantics:

var var1
if (!var1) {
  var1 = []
}

Edit I am not aware of how "strict mode" in Ed.5 influences the above.