Spaces, new lines, tabs, semicolons

Tip: Recall "Immediately-Invoked Function Expression (IIFE)" JavaScript concept.

Answer: Because of "Immediately-Invoked Function Expression (IIFE)" JavaScript interpreter treats like call of function with name d and paramater f+g. Like this:

x= d(f+g ).some_method();

There are "five problematic" characters to be careful with : "(" , "[" , "+" , "-", "/" . Never omit a semicolon before a statement beginning with the “five problematic” characters.

Just use "defensive semicolon" approach to avoid problems like this:

x= d 
;(f+g ).some_method()

Read it for details:
A lot of details: Immediately-Invoked Function Expression (IIFE)
wikipedia.org: Immediately-invoked function expression
pdf document: ECMAScript® 2015 Language Specification
Looks here in case you think that really know how it works