objectWrap — Prettier's one deliberate escape hatch, and how to switch it off

What the option does

Prettier normally decides layout entirely on its own: an object that fits within printWidth is collapsed to one line, and one that does not is expanded. Object literals are the exception.

With objectWrap: preserve — the default — Prettier looks at whether you put a line break between the { and the first key. If you did, the object stays expanded even when it would comfortably fit on one line. If you did not, normal width-based rules apply.

// newline after { — preserved
const a = {
  x: 1,
  y: 2,
};

// no newline after { — collapsed
const b = { x: 1, y: 2 };

Set objectWrap: collapse and the first form is reformatted to match the second. Prettier then owns the decision completely, as it does for every other construct.

Why the escape hatch exists

Objects carry meaning that width does not capture. A configuration object, a set of related constants, a table of test cases — these are often clearer one-key-per-line even when they are short, because the vertical form invites you to read them as a list rather than as a sentence.

Prettier's usual answer to that argument is no: the whole premise of the tool is that you stop making layout decisions. Object literals got an exception because the demand was overwhelming and the mechanism is cheap — a single newline, easy to add and easy to remove, with no configuration and no comment pragma.

It is worth knowing the same idea applies elsewhere under a different name. The so-called magic trailing comma — leaving a trailing comma in a list you want kept expanded — works on arrays, parameters and call arguments in the same spirit.

When to choose collapse

• •When you want formatting to be genuinely deterministic from the source text alone, so that two semantically identical files always format identically.
• •When your team keeps arguing about which objects deserve to be expanded — collapse ends the discussion by removing the choice.
• •When code is generated or heavily machine-edited, and incidental newlines from a codemod would otherwise be preserved as if they were intentional.

Bear in mind the cost: collapse will reformat a large number of small objects across an existing codebase, and some of them will read worse. Land it as its own commit.

Version note

objectWrap was added in Prettier 3.5. The preserve behaviour predates it by years — it was simply unconditional and had no option name. If you are on an older release the newline is still respected; you just cannot turn it off.

Common mistakes

• •Deleting the newline after { while tidying up, then wondering why an object collapsed.
• •Expecting the same preservation for arrays. Arrays use the trailing-comma mechanism instead.
• •Assuming Prettier output is a pure function of the AST. Under preserve it is not — this newline is part of the input.
• •Setting collapse and then trying to force expansion with comments. It will not work; the option is absolute.