Why JavaScript Statement "ga = ga || []" Works?

Question

Below javascript statements will cause error, if ga is not declared.

if (ga)
{
   alert(ga);
}

The error is:

ga is not defined

It looks undeclared variable cannot be recognized in bool expression. So, why below statement works?

var ga = ga || [];

To me, the ga is treated as bool value before "||". If it is false, expression after "||" is assigned to final ga.

Answer 1

null or defined are falsey values in javascript (implicitly evaluates to false.) the || operator returns the first value that does not evaluate to false.

var x = 0 || "" || null || "hello" || false; // x equals "hello"

On the other hand the && operator will return the first falsey value or the last value.

var y = "a string" && {"attr":"an object"} && false && ["array"]; 
// y equals false

var z = "a string" && {"attr":"an object"} && ["array"]; 
// z equals ["array"] 

Answer 2

Edit: You need to use var ga; first or var ga = ga || [];, because its declare ga first and assign values into it.

You could try this

var x = 1, y = x + 1;
alert([x,y]) // 1,2

You may notice, when y is declare, x is already declared and already assigned 1 to it.

So, in your case, when ga || [] assigns, ga is already declared and its valid variable.