type-flag

name

type-flag

description

Strongly-typed Node.js argv parser. Schema syntax (String/Number/Boolean, arrays, custom types, aliases, defaults, single-char names), return shape (flags/unknownFlags/positional args), flag forms (long/short/grouping, = : . delimiters, kebab↔camelCase), boolean negation via `--no-`, and the `ignore` callback for multi-command dispatch. Use when working with type-flag or building CLIs on top of it (e.g. cleye).

type-flag

Quick start

import { typeFlag } from 'type-flag'

const parsed = typeFlag({
    name: String,
    age: { type: Number, alias: 'a' },
})

// $ my-cli --name John -a 20
parsed.flags.name   // 'John'
parsed.flags.age    // 20
parsed.unknownFlags // {}
parsed._            // []  (positional args, with _['--'] for post-`--`)

Schema

Form	Example	Meaning
Shorthand	flag: String	Type only
Object	flag: { type: String, alias: 'f', default: 'x' }	Type + options
Array	flag: [String] or { type: [String] }	Collect multiple values
Custom type	flag: (raw: string) => MyType	Any (string) => T
Single-char name	x: Number	Matches -x, NOT --x

Type options:

Option	Type	Notes
type	TypeFunction | [TypeFunction]	Parser; wrap in [] for arrays
alias	string (1 char)	Forbidden when flag name is 1 char
default	T | (() => T)	Use a function for mutable defaults (objects/arrays)

Return shape

{
    flags:        { [name]: InferredType },
    unknownFlags: { [name]: (string | boolean)[] },  // not camelCased
    _:            string[] & { '--': string[] },      // positional; everything after `--` also in `_['--']`
}

Flag forms

Input	Behavior
--flag value / --flag=value	Long form
-f value / -f=value	Short form (alias or single-char name)
-abc	Group: each char matches an alias or single-char name independently
--some-flag	kebab-case → camelCase (someFlag) unless schema key is kebab
--flag:value / --flag.value	: and . also delimit values (useful for --define:K=V, --env.KEY=V)
-xvalue (concatenated)	⚠️ Parsed as GROUP, not x=value. Use -x value or -x=value.

Single-char flag names vs aliases

Both produce short flags that group identically (-ab, -av, mixed — all work). The difference is OUTPUT placement:

Pattern	When	Example
Single-char name ({ x: Number })	Short form IS the flag (coordinates, -h/--help as distinct entries)	Key lands in flags.x
Alias ({ verbose: { alias: 'v' } })	--verbose and -v should set the same value	Both forms feed flags.verbose

Single-char names match -x but NOT --x — this preserves schemas like { h: Boolean, help: Boolean } where -h and --help must stay distinct (rg-style short-vs-long help).

Boolean negation (opt-in)

typeFlag({ verbose: Boolean }, argv, { booleanNegation: true })
// --no-verbose  → flags.verbose = false
// --verbose --no-verbose → false (last wins)

Only affects Boolean-typed, schema-known flags. --no-X for non-boolean or unknown falls through to unknownFlags.

Explicit =false always works regardless of booleanNegation:

// --verbose=false → flags.verbose = false
// --verbose false → flags.verbose = true, `_` = ['false']  (space-separated is positional)

ignore callback

Skip parsing specific tokens (leave them in argv). Called for each token:

ignore?: (
    type: 'known-flag' | 'unknown-flag' | 'argument',
    flagOrArgv: string,
    value: string | undefined,
) => boolean | void

Two common patterns:

// 1. Leave unknown flags in argv (e.g. forward to child process)
typeFlag({}, argv, { ignore: type => type === 'unknown-flag' })

// 2. Stop parsing at first positional (Node-style subcommand dispatch)
let stop = false
typeFlag(schema, argv, {
    ignore: (type) => {
        if (stop) return true
        if (type === 'argument') { stop = true; return true }
    },
})

argv is mutated: type-flag removes parsed tokens. Pass process.argv.slice(2) (or a copy) to see what's left.

getFlag — one-off extraction

import { getFlag } from 'type-flag'

const name = getFlag('--name', String)          // single value
const age  = getFlag('-a,--age', Number)        // comma-separated names
const tags = getFlag('-t,--tag', [String])      // array form collects all

Same argv-mutation behavior as typeFlag.

Gotchas

Gotcha	Detail
Negative numbers look like flag groups	--num -123 is parsed as -1 -2 -3 (char group). Use --num=-123 or --num=-Inf.
argv is mutated	Parsed tokens are spliced out. Don't share argv with other parsers after.
Frozen argv throws	Object.freeze(argv) breaks the splice. Pass a mutable copy.
unknownFlags keys are raw	NOT camelCased — so you can distinguish --some-flag from --someFlag.
Reserved chars in names	\s, ., :, = forbidden in flag names (they're delimiters).
kebab schema key	If schema key is 'some-flag', only --some-flag / --someFlag both map to it, but output key stays kebab.
Default functions throw	A throwing default: () => ... propagates.
Custom-type errors wrap	Parser errors include the flag name in the thrown message.

Related

For full CLI framework with help generation and subcommands, see cleye (built on type-flag).