prettier/docs/rationale.md

id	title
rationale	Rationale

Prettier is an opinionated code formatter. This document explains some of its choices.

What Prettier is concerned about

Correctness

The first requirement of Prettier is to output valid code that has the exact same behavior as before formatting. Please report any code where Prettier fails to follow these correctness rules — that's a bug which needs to be fixed!

Strings

Double or single quotes? Prettier chooses the one which results in the fewest number of escapes. "It's gettin' better!", not 'It\'s gettin\' better!'. In case of a tie, Prettier defaults to double quotes (but that can be changed via the singleQuote option).

JSX always uses double quotes. JSX takes its roots from HTML, where the dominant use of quotes for attributes is double quotes. Browser developer tools also follow this convention by always displaying HTML with double quotes, even if the source code uses single quotes.

Prettier maintains the way your string is escaped. For example, "🙂" won't be formatted into "\uD83D\uDE42" and vice versa.

Empty lines

It turns out that empty lines are very hard to automatically generate. The approach that Prettier takes is to preserve empty lines the way they were in the original source code. There are two additional rules:

• Prettier collapses multiple blank lines into a single blank line.
• Empty lines at the start and end of blocks (and whole files) are removed. (Files always end with a single newline, though.)

Multi-line objects

By default, Prettier’s printing algorithm prints expressions on a single line if they fit. Objects are used for a lot of different things in JavaScript, though, and sometimes it really helps readability if they stay multiline. See object lists, nested configs, stylesheets and keyed methods, for example. We haven't been able to find a good rule for all those cases, so Prettier instead keeps objects multiline if there's a newline anywhere inside it in the original source code. A consequence of this is that long singleline objects are automatically expanded, but short multiline objects are never collapsed.

What Prettier is not concerned about

Prettier only prints code. It does not transform it. This is to limit the scope of Prettier. Let's focus on the printing and do it really well!

Here are a few examples of things that are out of scope for Prettier:

• Turning single- or double-quoted strings into template literals or vice versa.
• Adding/removing {} and return where they are optional.
• Turning ?: into if-else statements.