Question

Double quotes can be used to surround a literal string.

In this case, the string is a single space. And it is used to separate area from unit so they won't run together.

The double quotes used in this fashion is used as a space. It should also be noted that type of that expression will be of type String afterwards. You could write the same expression using ES6 template literals, which may come off as more intuitive; but the empty string method is something you will see in a lot of code:

return `${area} ${unit}1; 
//this will return the value of area and unit separated by a space, the same as:
return area + " " + unit;
//to see the type of this expression you can do something like, or use typeof() in your example code where
//the function will be returned:
console.log(typeof(5 + " " + "sq ft"); //returns type String

In PHP and Shell, the strings enclosed in single quotation is a string literal. That means what you wrote is what you get. Strings enclosed in double quotations, however, will be parsed. Hence it is advised to use single quotations in PHP or shell to maximize performance.

In JavaScript though and other languages such as Python, single and double quotation are treated the same - as string literal.