On this page

deno fmt, code formatting

Deno ships with a built-in code formatter that will auto-format the following files:

File Type Extension
JavaScript .js
TypeScript .ts
JSX .jsx
TSX .tsx
Markdown .md, .markdown
JSON .json
JSONC .jsonc

In addition, deno fmt can format code snippets in Markdown files. Snippets must be enclosed in triple backticks and have a language attribute.

# format all supported files in the current directory and subdirectories
deno fmt
# format specific files
deno fmt myfile1.ts myfile2.ts
# format all supported files in specified directory and subdirectories
deno fmt src/
# check if all the supported files in the current directory and subdirectories are formatted
deno fmt --check
# format stdin and write to stdout
cat file.ts | deno fmt -

Ignoring Code Jump to heading

Ignore formatting code by preceding it with a // deno-fmt-ignore comment in TS/JS/JSONC:

// deno-fmt-ignore
export const identity = [
    1, 0, 0,
    0, 1, 0,
    0, 0, 1,

Or ignore an entire file by adding a // deno-fmt-ignore-file comment at the top of the file.

In markdown you may use a <!-- deno-fmt-ignore --> comment or ignore a whole file with a <!-- deno-fmt-ignore-file --> comment. To ignore a section of markdown, surround the code with <!-- deno-fmt-ignore-start --> and <!-- deno-fmt-ignore-end --> comments.

Configuration Jump to heading

ℹ️ It is recommended to stick with default options.

Starting with Deno v1.14 a formatter can be customized using either a configuration file or following CLI flags:

  • --use-tabs - Whether to use tabs. Defaults to false (using spaces).

  • --line-width - The width of a line the printer will try to stay under. Note that the printer may exceed this width in certain cases. Defaults to 80.

  • --indent-width - The number of characters for an indent. Defaults to 2.

  • --no-semicolons - To not use semicolons except where necessary.

  • --single-quote - Whether to use single quote. Defaults to false (using double quote).

  • --prose-wrap={always,never,preserve} - Define how prose should be wrapped in Markdown files. Defaults to "always".

Note: In Deno versions < 1.31 you will have to prefix these flags with options- (ex. --options-use-tabs)