type-flag
Typed command-line arguments parser. Only 1.4 kB.
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🚀 Install
npm i type-flag
🚦 Quick start
type-flag offers a simple API to parse command-line arguments.
Let's say you want to create a script with the following usage:
$ my-script --name John --age 20
typeFlag
Here's how easy it is with type-flag:
import { typeFlag } from 'type-flag'
const parsed = typeFlag({
name: String,
age: {
type: Number,
alias: 'a'
}
})
console.log(parsed.flags.name)
console.log(parsed.flags.age)
You can also get unknown flags and arguments from the parsed
object:
console.log(parsed.unknownFlags)
console.log(parsed._)
getFlag
Want something even simpler?
type-flag also exports a getFlag
function that returns a single flag value.
import { getFlag } from 'type-flag'
const name = getFlag('--name', String)
const age = getFlag('-a,--age', Number)
console.log(name)
console.log(age)
These are quick demos but type-flag can do so much more:
- Accept multiple flag values
- Flag operators (e.g.
=
) for explicitly passing in a value - Parse unknown flags
- Parse alias groups (e.g.
-abc
)
Keep reading to learn more!
🧑💻 Usage
Defining flags
Pass in an object where the key is the flag name and the value is the flag type—a parser function that takes in a string and parses it to that type. Default JavaScript constructors should be able to cover most use-cases: String, Number, Boolean, etc.
The value can also be an object with the type
property as the flag type.
typeFlag({
stringFlag: String,
numberFlag: Number,
booleanFlag: Boolean,
stringFlag: {
type: String
}
})
Array type
To accept multiple values of a flag, wrap the type with an array:
const parsed = typeFlag({
myFlag: [String]
})
parsed.flags.myFlag
Aliases
Flags are often given single-character aliases for shorthand usage (eg. --help
to -h
). To give a flag an alias, use the object syntax and set the alias
property to a single-character name.
typeFlag({
myFlag: {
type: String,
alias: 'm'
}
})
parsed.flags.myFlag
Default values
Flags that are not passed in will default to being undefined
. To set a different default value, use the object syntax and pass in a value as the default
property. When a default is provided, the return type will reflect that instead of undefined
.
When using mutable values (eg. objects/arrays) as a default, pass in a function that creates it to avoid mutation-related bugs.
const parsed = typeFlag({
someNumber: {
type: Number,
default: 1
},
manyNumbers: {
type: [Number],
default: () => [1, 2, 3]
}
})
To get undefined
in the parsed flag type, make sure strict
or strictNullChecks
is enabled.
kebab-case flags mapped to camelCase
When passing in the flags, they can be in kebab-case and will automatically map to the camelCase equivalent.
const parsed = typeFlag({
someString: [String]
})
parsed.flags.someString
Unknown flags
When unrecognized flags are passed in, they are interpreted as a boolean, or a string if explicitly passed in. Unknown flags are not converted to camelCase to allow for accurate error handling.
const parsed = typeFlag({})
parsed.unknownFlags
Arguments
Arguments are values passed in that are not associated with any flags. All arguments are stored in the _
property.
Everything after --
(end-of-flags) is treated as an argument (including flags) and will be stored in the _['--']
property.
const parsed = typeFlag({
myFlag: [String]
})
parsed.flags.myFlag
parsed._
parsed._['--']
Flag value delimiters
The characters =
, :
and .
are reserved for delimiting the value from the flag.
$ node ./cli --flag=value --flag:value --flag.value
This allows for usage like --flag:key=value
or --flag.property=value
to be possible.
Mutated argv array
When type-flag
iterates over the argv array, it removes the tokens it parses out via mutation.
By default, type-flag
works on a new copy of process.argv.slice(2)
so this doesn't have any side-effects. But if you want to leverage this behavior to extract certain flags and arguments, you can pass in your own copy of process.argv.slice(2)
.
This may be useful for filtering out certain flags before passing down the argv
to a child process.
Ignoring unknown flags
Sometimes it may be undesirable to parse unknown flags. In these cases, you can ignore them so they're left unparsed in the argv
array.
const argv = process.argv.slice(2)
const parsed = typeFlag(
{},
argv,
{
ignore: type => type === 'unknown-flag'
}
)
parsed._
argv
Ignoring everything after the first argument
Similarly to how Node.js only reads flags passed in before the first argument, type-flag can be configured to ignore everything after the first argument.
const argv = process.argv.slice(2)
let stopParsing = false
const parsed = typeFlag(
{
myFlag: [Boolean]
},
argv,
{
ignore(type) {
if (stopParsing) {
return true
}
const isArgument = type === 'argument'
if (isArgument) {
stopParsing = isArgument
return stopParsing
}
}
}
)
parsed.flags.myFlag
argv
👨🏻🏫 Examples
Custom flag type
Basic types can be set using built-in functions in JavaScript, but sometimes you want to a new type, narrow the type, or add validation.
To create a new type, simply declare a function that accepts a string argument and returns the parsed value with the expected type.
In this example, the size
flag is enforced to be either small
, medium
or large
.
const possibleSizes = ['small', 'medium', 'large'] as const
type Sizes = typeof possibleSizes[number]
function Size(size: Sizes) {
if (!possibleSizes.includes(size)) {
throw new Error(`Invalid size: "${size}"`)
}
return size
}
const parsed = typeFlag({
size: Size
})
parsed
resolves to the following type:
type Parsed = {
flags: {
size: 'small' | 'medium' | 'large' | undefined
}
}
Optional value flag
To create a string flag that acts as a boolean when nothing is passed in, create a custom type that returns both types.
function OptionalString(value: string) {
if (!value) {
return true
}
return value
}
const parsed = typeFlag({
string: OptionalString
})
parsed.flags.string
parsed.flags.string
Accepting flag values with =
in it
In use-cases where flag values contain =
, you can use :
instead. This allows flags like --define:K=V
.
const parsed = typeFlag({
define: String
})
parsed.flags.define
Dot-nested flags
type Environment = {
TOKEN?: string
CI?: boolean
}
function EnvironmentObject(value: string): Environment {
const [propertyName, propertyValue] = value.split('=')
return {
[propertyName]: propertyValue || true
}
}
const parsed = typeFlag({
env: [EnvironmentObject]
})
const env = parsed.flags.env.reduce(
(agg, next) => Object.assign(agg, next),
{}
)
env
Inverting a boolean
To invert a boolean flag, false
must be passed in with the =
operator (or any other value delimiters).
const parsed = typeFlag({
booleanFlag: Boolean
})
parsed.flags.booleanFlag
Without explicitly specfying the flag value via =
, the false
will be parsed as a separate argument.
parsed.flags.booleanFlag
parsed._
Counting flags
To create an API where passing in a flag multiple times increases a count (a pretty common one is -vvv
), you can use an array-boolean type and count the size of the array:
const parsed = typeFlag({
verbose: {
type: [Boolean],
alias: 'v'
}
})
parsed.flags.verbose.length
⚙️ API
typeFlag(flagSchema, argv, options)
Returns an object with the shape:
type Parsed = {
flags: {
[flagName: string]: InferredType
}
unknownFlags: {
[flagName: string]: (string | boolean)[]
}
_: string[]
}
flagSchema
Type:
type TypeFunction = (argvValue: any) => any
type FlagSchema = {
[flagName: string]: TypeFunction | [TypeFunction] | {
type: TypeFunction | [TypeFunction]
alias?: string
default?: any
}
}
An object containing flag schema definitions. Where the key is the flag name, and the value is either the type function or an object containing the type function and/or alias.
argv
Type: string[]
Default: process.argv.slice(2)
The argv array to parse. The array is mutated to remove the parsed flags.
options
Type:
type Options = {
ignore?: (
type: 'known-flag' | 'unknown-flag' | 'argument',
flagOrArgv: string,
value: string | undefined,
) => boolean | void
}
getFlag(flagNames, flagType, argv)
flagNames
Type: string
A comma-separated list of flag names to parse.
flagType
Type:
type TypeFunction = (argvValue: any) => any
type FlagType = TypeFunction | [TypeFunction]
A function to parse the flag value. Wrap the function in an array to retrieve all values.
argv
Type: string[]
Default: process.argv.slice(2)
The argv array to parse. The array is mutated to remove the parsed flags.