TYPESCRIPT

typescript default value if null

/* Use || operator to assign a default value whence the first variable
evaluates to a 'falsy' value
	- Example with strings (you can replace string with any type T) */

let x: string | null = null;
let y: string = x || "My default value"

typescript default value null or undefined

The value undefined means value is not assigned 
  & you don’t know its value.
  It is an unintentional absence of value.
  It means that a variable has been declared but has not yet
  been assigned a value.

The value null indicates that you know that the field does not
  have a value. It is an intentional absence of value.