JavaScript formatter that moves whitespace only

It can give you the layout back, and no tool without a source map can give you the names. Run a 185 KB framework chunk through it and you get 11,498 indented lines in about 50 ms, with every function body, branch and chained call where you can see it — which is usually enough to answer why is this throwing. What you will still be reading is e, t, n and r, because the build step that shortened those identifiers threw the originals away and only the .map file remembers them. If your bundle ships with one, your browser devtools will do a better job than any text box; if it does not, this is what is left.

No, and avoiding that is the hardest single thing about reading JavaScript. A forward slash starts a regex in one position and divides in another, so a/b/c is arithmetic while the same characters after an equals sign are a pattern — the language genuinely cannot be tokenised without tracking what came before. The reader keeps that context: it looks at the previous significant token, keeps a stack of the parentheses that belong to if, while and for heads, and refuses to run a pattern scan past the end of a line. Whatever it decides is a regex is copied out character for character, flags included.

It carries each JSX region across untouched, on the line where the surrounding statement leaves it. Element markup is scanned as one opaque unit from the opening angle bracket to its matching close, so attributes, expression containers and text are exactly as you wrote them — but they are not re-indented internally, and a component whose render is one long tree will come back as one long line. If the reader cannot find a coherent element it falls back to treating the angle bracket as a less-than operator, which is the right guess for ordinary code and the reason plain arithmetic never gets mistaken for markup.

Not as types, and the output shows it. Annotations survive because every character survives, but the printer sees Array<number> as three tokens with two comparison operators in the middle and spaces them accordingly, which is legal TypeScript and ugly TypeScript. Decorators stay on the line with the class they decorate. If you are formatting .ts or .tsx as part of your normal workflow, an editor plugin that parses the type grammar will treat you better; this is for the moment when you have a lump of unfamiliar JavaScript and no toolchain in front of you.

Neither — not one is inserted, and not one is deleted. That is a safety decision rather than a stylistic one, because JavaScript ends statements at newlines under rules that are easy to get wrong: a return followed by a line break returns undefined no matter what sits on the next line, and the same applies to throw, break, continue and yield. The printer keeps the newline after all five, so a value can never be dragged up onto a return that was meant to be bare. Whether you write semicolons is left exactly as you decided.

All three come through byte for byte. The inside of a template literal is content, including the newlines that a tagged template may be counting on, so nothing indented, re-wrapped or re-quoted ever happens in there — the embedded ${…} expressions are left as written too. String bodies are only ever re-quoted if you ask for it and the literal has no quote character and no backslash inside. Every comment is kept: a trailing one stays at the end of its line, a block comment keeps its internal line breaks, and the count under the tool tells you how many came through.

Prettier throws your file away and prints a new one from the syntax tree; this re-spaces the tokens of the file you pasted. That difference decides everything else. Prettier can reflow a long expression across lines intelligently, normalise your parentheses and reprint JSX properly, and it needs a valid parse of the whole file to do any of it. This one cannot reflow an arbitrary expression — but it will still lay out a file with a syntax error further down, and it can promise that the only characters that moved were whitespace. Both are useful; they are useful at different moments.