jsflags

An extremely simple yet flexible JavaScript library for parsing command-line flags, inspired by the golang "flags" library.

Features

• No dependencies.
• Easy and flexible validation API.
• TypeScript and JavaScript support.
• CJS and ESM support.

Basic Usage

import FlagSet, { integer, boolean, defaultValue } from "jsflags"
import { parseNodejs } from "jsflags/node"

const flags = new FlagSet()
const portRef = flags.flag(defaultValue(integer, 3000), "port", "Specify what port to host on.")
const verboseRef = flags.flag(boolean, "verbose", "Enable verbose logging.")

parseNodejs(flags)
console.log(portRef.value, verboseRef.value)

Full Usage

import FlagSet, { integer, string, boolean, defaultValue, single } from "jsflags"
import { parseNodejs } from "jsflags/node"

const flags = new FlagSet((values) => {
  return single(values)
})

const portRef = flags.flag(integer, "port", "Specify the port to host on.")
const nameRef = flags.flag(string, "name", "Specify the name of the application.")
const verboseRef = flags.flag(boolean, "verbose", "Enable verbose logging.")
const multipleRef = flags.flag(multiple)

const positional = parseNodejs(flags)
console.log(portRef.value, nameRef.value, verboseRef.value, positional)
// $ application --port 200 --name "some fancy name" positional
// 200 "some fancy name" false "positional"
// 
// $ application --port 4200 --name "some fancy name" --verbose arg
// 4200 "some fancy name" true "arg"

console.log(flags.help())
// Usage:
//   	Positionals:	single
//
//   	-port	integer	Specify the port to host on.
//
//   	-name	string	Specify the name of the application.
//
//   	-verbose	boolean (default: false)	Enable verbose logging.

• Each flag returns a "reference", an object { value: ... } whose property value is set to the value of the flag.
• A reference's "value" is set to null before parseFlags(...) is called.
• Flag names must only contain letters, numbers, _ and -. (Regex: [\w-])
• Quoted flag values must always use double quotes.

The following formats are accepted for arguments:

| Args | Result | | --- | --- | | -flag | [""] | | -flag value | ["value"] | | -flag=value | ["value"] | | -flag=v1 -flag v2 -flag | ["v1", "v2", ""] | | --flag | [""] | | --flag value | ["value"] | | --flag=value | ["value"] | | --flag=v1 --flag v2 --flag | ["v1", "v2", ""] |

Custom validation and array values

All flags possess a "validator" to verify and transform the raw string values of the flag into the final value in the reference.

// T is the type in the reference.
export type Validator<T> = (values: string[]) => T;

A validator that does nothing (like flag((values) => values, ...)) allows for multiple (or zero) values for a given flag. This is often undesired, so the single validator is used by default on most of the built-in validators.

The single validator throws an error if there is not exactly one value passed in. (Do note that the string validator is just an alias for single as the two do the exact same thing.)

This means you can pass in your own validators to validate a custom type.

function json(values: string[]) {
  return JSON.parse(single(values))
}
const jsonRef = flag(json, "json", "...")
parseFlags(process.argv.slice(2)) // $ application --json {"json": [1, "oh no", 3]}
console.log(jsonRef.value) // {json: [1, "oh no", 3]}

The type of a value in the help message is simply the name of the validator function. In the example above, it would show up like -json json ....

If you want a list of typed values. multiple(validator) returns a validator that maps a given validator over each element in the values passed in, it then returns an array of the values returned.

const portRef = flag(multiple(integer), "port", "")
parseFlags(process.argv.slice(2)) // $ application --port 323 --port=424
console.log(portRef.value) // [323, 424]