javascript: define a variable if it doesn't exist

if (typeof variable === 'undefined') {
    // variable is undefined
    // eg:
    // var variable = "someValue";
}

It would be a good coding practice in this case to use the ternary operator. Also you don't need to have three equal signs when comparing with typeof. This is the most concise solution:

b = typeof(b) == 'undefined' ? 0 : b;

This hopefully will save your hands some time.

I think your posted code should work. Unless your original value is 0.

The problem is somewhere else.

I'm guessing you defined 'embed_BackgroundColor' out of the scope of your code. And when you run your code, that variable is undefined with in the scope of your code, and will be assigned the default value.

Here is an example:

var embed_BackgroundColor = "#FF0000";

(function(){
  if(!embed_BackgroundColor) {
    var embed_BackgroundColor;
    embed_BackgroundColor = "#F4F4F4";
  }
  alert(embed_BackgroundColor); // will give you #F4F4F4
})();

alert(embed_BackgroundColor); // will give you #FF0000;

I prefer a general solution in PHP-like style:

function isset(x) { return typeof(x)!='undefined'; }

As of ES2020 you can now use the nullish coalescing operator ??. This avoids reassigning falsy values to the default value.

var embed_BackgroundColor = embed_BackgroundColor ?? "#F4F4F4"

Note that if you are doing a oneliner like this you need to use var; let and const won't let you do it in one line.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing_operator

Cleanest way in 2022 ECMA syntax!!

If you want to overwrite "nullish" values (see ref below), assign using let might_exist ||= default or pass value with (might_exist || default). (The parentheses are only there for clarity and to avoid operator precedence problems, delete if you'd rather.)

If you want to keep "nullish" values, assign using let might_exist ??= default or pass value with (might_exist ?? default).

On being nullish: ?? will only overwrite null and undefined, whereas || will also overwrite "nullish" values like '' and 0.

You're all very welcome, it's about time I gave back a lil

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_OR_assignment

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_nullish_assignment

if(embed_BackgroundColor == "" || embed_BackgroundColor == 'undefined' || embed_BackgroundColor == null){
}