Deno Style Guide

If the code originates elsewhere, ensure that the file has the proper copyright headers. We only allow MIT, BSD, and Apache licensed code.

Use underscores, not dashes in filenames.

Example: Use file_server.ts instead of file-server.ts.

Each module should contain or be accompanied by tests for its public functionality.

TODO Comments

TODO comments should usually include an issue or the author’s github username in parentheses. Example:

// TODO(ry): Add tests.
// TODO(#123): Support Windows.
// FIXME(#349): Sometimes panics.

Be explicit even when it means more code.

There are some situations where it may make sense to use such techniques, but in the vast majority of cases it does not.

Inclusive code

Please follow the guidelines for inclusive code outlined at .

Follow Rust conventions and be consistent with existing code.

TypeScript

The TypeScript portion of the code base is the standard library std.

Use the term “module” instead of “library” or “package”.

For clarity and consistency avoid the terms “library” and “package”. Instead use “module” to refer to a single JS or TS file and also to refer to a directory of TS/JS code.

Deno does not treat “index.js” or “index.ts” in a special way. By using these filenames, it suggests that they can be left out of the module specifier when they cannot. This is confusing.

If a directory of code needs a default entry point, use the filename mod.ts. The filename mod.ts follows Rust’s convention, is shorter than index.ts, and doesn’t come with any preconceived notions about how it might work.

Exported functions: max 2 args, put the rest into an options object.

A function that is part of the public API takes 0-2 required arguments, plus (if necessary) an options object (so max 3 total).
Optional parameters should generally go into the options object.

An optional parameter that’s not in an options object might be acceptable if there is only one, and it seems inconceivable that we would add more optional parameters in the future.
The ‘options’ argument is the only argument that is a regular ‘Object’.

Other arguments can be objects, but they must be distinguishable from a ‘plain’ Object runtime, by having either:
- a distinguishing prototype (e.g. Array, Map, Date, class MyThing).
- a well-known symbol property (e.g. an iterable with Symbol.iterator).
This allows the API to evolve in a backwards compatible way, even when the position of the options object changes.

// BAD: optional parameters not part of options object. (#2)
export function resolve(
  hostname: string,
  family?: "ipv4" | "ipv6",
  timeout?: number,
): IPAddress[] {}
// GOOD.
export interface ResolveOptions {
  family?: "ipv4" | "ipv6";
  timeout?: number;
}
export function resolve(
  hostname: string,
  options: ResolveOptions = {},
): IPAddress[] {}

export interface Environment {
  [key: string]: string;
}
// BAD: `env` could be a regular Object and is therefore indistinguishable
// from an options object. (#3)
export function runShellWithEnv(cmdline: string, env: Environment): string {}
// GOOD.
  env: Environment;
}
export function runShellWithEnv(
  cmdline: string,
  options: RunShellOptions,
): string {}

// BAD: more than 3 arguments (#1), multiple optional parameters (#2).
export function renameSync(
  oldname: string,
  newname: string,
  replaceExisting?: boolean,
  followLinks?: boolean,
) {}
// GOOD.
interface RenameOptions {
  replaceExisting?: boolean;
  followLinks?: boolean;
}
export function renameSync(
  oldname: string,
  newname: string,
  options: RenameOptions = {},
) {}

// BAD: too many arguments. (#1)
export function pwrite(
  fd: number,
  buffer: TypedArray,
  offset: number,
  length: number,
  position: number,
) {}
// BETTER.
export interface PWrite {
  fd: number;
  buffer: TypedArray;
  length: number;
  position: number;
}
export function pwrite(options: PWrite) {}

Whenever you are using interfaces that are included in the arguments of an exported member, you should export the interface that is used. Here is an example:

Minimize dependencies; do not make circular imports.

Although std has no external dependencies, we must still be careful to keep internal dependencies simple and manageable. In particular, be careful not to introduce circular imports.

Sometimes there may be situations where an internal module is necessary but its API is not meant to be stable or linked to. In this case prefix it with an underscore. By convention, only files in its own directory should import it.

Use JSDoc for exported symbols.

We strive for complete documentation. Every exported symbol ideally should have a documentation line.

If possible, use a single line for the JSDoc. Example:

/** foo does bar. */
export function foo() {
  // ...
}

It is important that documentation is easily human readable, but there is also a need to provide additional styling information to ensure generated documentation is more rich text. Therefore JSDoc should generally follow markdown markup to enrich the text.

Code string literals should be braced with the back-tick (`) instead of quotes. For example:

/** Import something from the `deno` module. */

Do not document function arguments unless they are non-obvious of their intent (though if they are non-obvious intent, the API should be considered anyways). Therefore @param should generally not be used. If @param is used, it should not include the type as TypeScript is already strongly typed.

/**
 * Function with non obvious param.
 * @param foo Description of non obvious parameter.
 */

Vertical spacing should be minimized whenever possible. Therefore single line comments should be written as:

/** This is a good single line JSDoc. */

And not:

/**
 * This is a bad single line JSDoc.
 */

Code examples should utilize markdown format, like so:


Code examples should not contain additional comments and must not be indented. It is already inside a comment. If it needs further comments it is not a good example.
### Resolve linting problems using directives
Currently, the building process uses `dlint` to validate linting problems in the code. If the task requires code that is non-conformant to linter use `deno-lint-ignore <code>` directive to suppress the warning.

// deno-lint-ignore no-explicit-any let x: any;


This ensures the continuous integration process doesn't fail due to linting problems, but it should be used scarcely.
### Each module should come with a test module.
Every module with public functionality `foo.ts` should come with a test module `foo_test.ts`. A test for a `std` module should go in `std/tests` due to their different contexts, otherwise it should just be a sibling to the tested module.
### Unit Tests should be explicit.
For a better understanding of the tests, function should be correctly named as its prompted throughout the test command. Like:

test myTestFunction … ok


Example of test:

import { assertEquals } from ““; import { foo } from “./mod.ts”;

Deno.test(“myTestFunction”, function () { assertEquals(foo(), { bar: “bar” }); });


### Top level functions should not use arrow syntax.
Top level functions should use the `function` keyword. Arrow syntax should be limited to closures.
Bad:

export const foo = (): string => { return “bar”; };


Good:

export function foo(): string { return “bar”; }

Maintain browser compatibility for such a module by either not using the global Deno namespace or feature-testing for it. Make sure any new dependencies are also browser compatible.

Style guide