arglet

Arglet

A tiny helper that lets your CLI args lead

Overview

Arglet is a small, joyful utility for CLI tools that helps merge configuration from CLI arguments.

Arglet is designed to be:

• 🌱 lightweight and dependency-free
• 🧠 predictable and easy to reason about
• ✨ TypeScript-first with great IntelliSense
• 🔒 schema-driven and type-respecting
• 🔧 parser-agnostic

Arglet uses your configuration object as the source of truth. Only keys defined in that object are allowed, and values are parsed according to their existing type.

Installation

npm install arglet
# or
pnpm add arglet
# or
yarn add arglet

Quick Start

Get a fully-typed CLI configuration in one line.

import arglet from "arglet";

const config = arglet({
  input: "src",
  output: "dist",
  watch: false,
});

node cli.js --input=lib --watch

Output:

{
  input: "lib",
  output: "dist",
  watch: true
}

Usage

Basic usage

Define a configuration object and let Arglet update it using CLI arguments.

import arglet from "arglet";

const config = arglet({
  name: "sriman",
  age: 23,
  debug: false,
});

Run your script:

node cli.js --name tene --age 25 --debug

Result:

{
  name: "tene",
  age: 25,
  debug: true
}

Arglet respects the type defined in your config. Since age is a number in the schema, "25" is automatically cast to 25.

Type behavior (schema-driven parsing)

Arglet parses values strictly based on the type in your configuration object.

Schema Type	CLI Input	Result Type
boolean	--flag	true
boolean	--no-flag	false
number	--port=8080	number
string	--port=8080	"8080"
string[]	--tags=a,b	string[]
number[]	--ids=1,2,3	number[]

If the type is undefined or null, Arglet preserves the raw string value.

Boolean flags

Boolean options support implicit enable/disable flags.

const config = arglet({
  verbose: false,
  cache: true,
});

--verbose        # sets verbose → true
--no-cache       # sets cache → false

❗ Boolean shortcuts are only allowed for boolean options. Using --flag or --no-flag on non-boolean keys throws an error.

Explicit values

Non-boolean options must receive a value.

const config = arglet({
  port: 3000,
});

--port 8080
# or
--port=8080

If a value cannot be parsed according to the schema type, Arglet throws an error.

Example:

--port hello

❌ Throws:

--port expects a number

Array values

Provide multiple values using a separator (, by default).

const config = arglet({
  ids: [] as number[],
});

--ids=1,2,3

Result:

{
  ids: [1, 2, 3];
}

If your schema is:

const config = arglet({
  ids: [] as string[],
});

Then:

--ids=1,2,3

Result:

{
  ids: ["1", "2", "3"];
}

You can customize the separator:

arglet({ ids: [] }, { arraySeparator: "|" });

--ids=1|2|3

Nested configuration (dot notation)

Arglet supports deep configuration using dot paths.

const config = arglet({
  server: {
    host: "localhost",
    port: 3000,
  },
});

--server.host=0.0.0.0 --server.port=8080

Arglet will update nested properties while preserving types.

Only existing paths are allowed — unknown nested keys are ignored.

Unknown flags

Flags that do not exist in your configuration object are ignored.

--unknown

Ignored (unless debug mode is enabled).

Custom arguments (testing & programmatic use)

You can pass arguments directly (useful for tests or programmatic usage).

const config = arglet({ debug: false }, ["--debug"]);

Debug mode

Enable debug output to see how arguments are parsed and applied.

arglet({ debug: false }, { debug: true });

This logs:

• ignored flags
• boolean inference decisions
• final resolved configuration

Error handling

Arglet is intentionally strict.

The following will throw errors:

--age            # age is not boolean
--no-name        # name is not boolean
--port hello     # port expects number

This keeps CLI behavior predictable and safe.

Example CLI

import arglet from "arglet";

const config = arglet({
  input: "src",
  output: "dist",
  watch: false,
});

console.log(config);

node cli.js --input=lib --watch

Philosophy

Arglet is not a full argument parser. It assumes you already control argument shape.

Its job is simple:

• Treat your config as the schema
• Merge CLI flags safely
• Respect types
• Fail loudly when misused

Predictable input → predictable output.

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