JavaScript shorthand ternary operator

var startingNumber = startingNumber || 1;

Something like that what you're looking for, where it defaults if undefined?

var foo = bar || 1; // 1
var bar = 2;
foo = bar || 1;     // 2

By the way, this works for a lot of scenarios, including objects:

var foo = bar || {}; // secure an object is assigned when bar is absent

|| will return the first truthy value it encounters, and can therefore be used as a coalescing operator, similar to C#'s ??

startingNum = startingNum || 1;

Yes, there is:

var startingNum = startingNum || 1;

In general, expr1 || expr2 works in the following way (as mentioned by the documentation):

Returns expr1 if it can be converted to true; otherwise, returns expr2. Thus, when used with Boolean values, || returns true if either operand is true; if both are false, returns false.

With the addition of ES2020:

New w/Nullish Coalescence: const difficulty = var?.nest[i]?.prop ?? false

Older Operation: const difficulty = var.nest[i].prop ? var.nest[i].prop : false

The question mark before the property will first check if the object even exists (if you aren't sure it will: like in API data) and, if an object is missing, it will return undefined

The ?? checks if the value on the left is null or undefined and, if it is, will return a supplied value on the right.