code-style: Is inline initialization of JS objects ok?

Question

I often find myself using inline initialization (see example below), especially in a switch statement when I don't know which case loop will hit. I find it easier to read than if statements.

But is this good practice or will it incur side-effects or a performance hit?

for (var i in array) {
    var o = o ? o : {};  // init object if it doesn't exist
    o[array[i]] = 1;     // add key-values
}

Is there a good website to go to get coding style tips?

Answer 1

Why not just declare it outside the loop?

var o = {};
for (var i in array) {
  o[array[i]] = 1;
}

Otherwise no, I don't see a problem with what you're doing.

Answer 2

Another commonly used pattern to do the same, is the use of the Logical OR || operator (a little bit more readable than your ternary IMHO):

//...
var obj = o || {};

This operator will return its second operand if the first one evaluates to false, otherwise it will return the first one.

Is safe to use it when you expect an object, since those falsy values are null, undefined, NaN, 0, a zero-length string, and of course false.

I find it useful to set default values on function arguments, when of course any of the falsy values are expected as valid by the function:

function test (arg1) {
  arg1 = arg1 || "default value";
  //..
}