What is a concise and robust way to write these statements in JavaScript?

Question

I have an <iframe> that uses some variables that are set in an object literal in the parent document. These variables are optional; if they're set, I want to use their values, assigning them to a more succinct global variable; if not, I want to call the function error().

First of all, is this ternary operator statement valid and effective for what I want to do? Are there any potential traps here?

parent.settings.account ? var account = parent.settings.account : error();

Secondly, is there a more concise way to write that statement?

A few of these variables are optional, but if defined, have a range of acceptable values. What's a robust way to handle this? Something like this?

if (parent.settings.polling_interval && parent.settings.polling_interval >= 1000 && parent.settings.polling_interval <= 5000) {
    var polling_interval = parent.settings.polling_interval;
} else {
  var polling_interval = 3000; // default
}

Basically, if it's undefined or out of range, I want to use a default value.

Can I rely on just a conditional statement like:

if (parent.settings.polling_interval) { ... }

or do I need to check if it's undefined, like this?

if (typeof parent.settings.polling_interval !== 'undefined') { ... }

Is there something more robust and/or concise? Thanks for any advice.

Answer 1

This is wrong:

parent.settings.account ? var account = parent.settings.account : error();

You meant to say:

var account = parent.settings.account ? parent.settings.account : error();

Or even:

var account = parent.settings.account || error();

Answer 2

var account = parent.settings.account;
if (!account)
    error();

if ((p = parent.settings.polling_interval) && p >= 1000 && p <= 5000) {
   ...
}

var polling_interval = (p = parent.settings.polling_interval) && p >= 1000 && p <= 5000 ? p : 3000;

Answer 3

[1]

parent.settings.account ? ( var account = parent.settings.account ) : error();

This will raise a SyntaxError since you can't define a variable in that manner.

If error() raises an Exception / throws an Error then you can do:

    if ( !parent.settings.account ) {
        error()
    }

// continue code

As previously stated you can also assign the variable to the property:

var account = parent.settings.account || error();

[2]

  var pollingInterval = parent.settings.polling_interval &&
 parent.settings.polling_interval >= 1000 &&
 parent.settings.polling_interval <= 5000 ?
 parent.settings.polling_interval : 3000;

[3] You can use if ( parent.foo ) and if foo is undefined no strict error will be thrown but it will just equal undefined, assuming parent is always defined.

If settings is defined and polling_interval is not, same case. If you're sure the property is set and the settings object will exist, then you can use

if ( parent.settings.polling_interval ) { }

It would help stating what error() actually does.