Add isRequired flag option

fixture-required-function.js

@@ -0,0 +1,24 @@
+#!/usr/bin/env node
+'use strict';
+const meow = require('.');
+
+const cli = meow({
+	description: 'Custom description',
+	help: `
+		Usage
+		  foo <input>
+  `,
+	flags: {
+		trigger: {
+			type: 'boolean',
+			alias: 't'
+		},
+		withTrigger: {
+			type: 'string',
+			isRequired: (flags, _) => {
+				return flags.trigger;
+			}
+		}
+	}
+});
+console.log(`${cli.flags.trigger},${cli.flags.withTrigger}`);

fixture-required.js

@@ -0,0 +1,26 @@
+#!/usr/bin/env node
+'use strict';
+const meow = require('.');
+
+const cli = meow({
+	description: 'Custom description',
+	help: `
+		Usage
+		  foo <input>
+  `,
+	flags: {
+		test: {
+			type: 'string',
+			alias: 't',
+			isRequired: true
+		},
+		number: {
+			type: 'number',
+			isRequired: true
+		},
+		notRequired: {
+			type: 'string'
+		}
+	}
+});
+console.log(`${cli.flags.test},${cli.flags.number}`);

index.d.ts

@@ -2,11 +2,13 @@ import {PackageJson} from 'type-fest';
 
 declare namespace meow {
 	type FlagType = 'string' | 'boolean' | 'number';
+	type IsRequiredPredicate = (flags: AnyFlags, input: string[]) => boolean;
 
 	interface Flag<Type extends FlagType, Default> {
 		readonly type?: Type;
 		readonly alias?: string;
 		readonly default?: Default;
+		readonly isRequired?: boolean | IsRequiredPredicate
 	}
 
 	type StringFlag = Flag<'string', string>;
@@ -24,6 +26,7 @@ declare namespace meow {
 		- `type`: Type of value. (Possible values: `string` `boolean` `number`)
 		- `alias`: Usually used to define a short flag alias.
 		- `default`: Default value when the flag is not specified.
+		- `isRequired`: Boolean or Function that specifies if this flag is required.
 
 		@example
 		```
@@ -32,6 +35,9 @@ declare namespace meow {
 				type: 'string',
 				alias: 'u',
 				default: 'rainbow'
+				isRequired: (flags, input) => {
+					if (flags.otherFlag) return true;
+				}
 			}
 		}
 		```

index.js

@@ -51,6 +51,16 @@ const meow = (helpText, options) => {
 		options.flags
 	) : options.flags;
 
+	// Get a list of required flags
+	const requiredFlags = [];
+	if (typeof options.flags !== 'undefined') {
+		for (const flagName of Object.keys(options.flags)) {
+			if (options.flags[flagName].isRequired) {
+				requiredFlags.push(flagName);
+			}
+		}
+	}
+
 	let minimistoptions = {
 		arguments: options.input,
 		...minimistFlags
@@ -118,6 +128,30 @@ const meow = (helpText, options) => {
 		}
 	}
 
+	// Get a list of missing flags, that are required
+	const missing = [];
+	for (const requiredFlag of requiredFlags) {
+		let requiredByFunction = true;
+
+		if (typeof options.flags[requiredFlag].isRequired === 'function') {
+			requiredByFunction = options.flags[requiredFlag].isRequired(flags, input);
+		}
+
+		if (typeof flags[requiredFlag] === 'undefined' && requiredByFunction) {
+			missing.push({key: requiredFlag, ...options.flags[requiredFlag]});
+		}
+	}
+
+	// Print error message for missing flags
+	if (missing.length > 0) {
+		console.log(`Missing required option${missing.length > 1 ? 's' : ''}`);
+		for (const flag of missing) {
+			console.log(`\t--${flag.key}${flag.alias ? `, -${flag.alias}` : ''}`);
+		}
+
+		process.exit(2);
+	}
+
 	return {
 		input,
 		flags,

readme.md

@@ -97,6 +97,7 @@ The key is the flag name and the value is an object with any of:
 - `type`: Type of value. (Possible values: `string` `boolean` `number`)
 - `alias`: Usually used to define a short flag alias.
 - `default`: Default value when the flag is not specified.
+- `isRequired`: Boolean or Function that specified is this flag is required.
 
 Example:
 
@@ -106,6 +107,9 @@ flags: {
 		type: 'string',
 		alias: 'u',
 		default: 'rainbow'
+		isRequired: (flags, input) => {
+			if (flags.otherFlag) return true;
+		}
 	}
 }
 ```

test.js

@@ -80,6 +80,72 @@ test('spawn cli and test input flag', async t => {
 	t.is(stdout, 'bar');
 });
 
+test('spawn cli and test required flag with missing args', async t => {
+	try {
+		await execa('./fixture-required.js', []);
+	} catch (error) {
+		const {stdout, message} = error;
+		t.regex(message, /Command failed with exit code 2/);
+		t.regex(stdout, /Missing required option/);
+		t.regex(stdout, /--test, -t/);
+		t.regex(stdout, /--number/);
+		t.notRegex(stdout, /--notRequired/);
+	}
+});
+
+test('spawn cli and test required flag with all args given', async t => {
+	const {stdout} = await execa('./fixture-required.js', [
+		'-t',
+		'test',
+		'--number',
+		'6'
+	]);
+	t.is(stdout, 'test,6');
+});
+
+test('spawn cli and test required flag with empty string', async t => {
+	try {
+		await execa('./fixture-required.js', ['--test', '']);
+	} catch (error) {
+		const {stdout, message} = error;
+		t.regex(message, /Command failed with exit code 2/);
+		t.regex(stdout, /Missing required option/);
+		t.notRegex(stdout, /--test, -t/);
+	}
+});
+
+test('spawn cli and test required flag with empty number', async t => {
+	try {
+		await execa('./fixture-required.js', ['--number']);
+	} catch (error) {
+		const {stdout, message} = error;
+		t.regex(message, /Command failed with exit code 2/);
+		t.regex(stdout, /Missing required option/);
+		t.regex(stdout, /--number/);
+	}
+});
+
+test('spawn cli and test required (specified as function) flag with without arguments', async t => {
+	const {stdout} = await execa('./fixture-required-function.js', []);
+	t.is(stdout, 'false,undefined');
+});
+
+test('spawn cli and test required (specified as function) flag with trigger only', async t => {
+	try {
+		await execa('./fixture-required-function.js', ['--trigger']);
+	} catch (error) {
+		const {stdout, message} = error;
+		t.regex(message, /Command failed with exit code 2/);
+		t.regex(stdout, /Missing required option/);
+		t.regex(stdout, /--withTrigger/);
+	}
+});
+
+test('spawn cli and test required (specified as function) flag with trigger and dynamically required option', async t => {
+	const {stdout} = await execa('./fixture-required-function.js', ['--trigger', '--withTrigger', 'specified']);
+	t.is(stdout, 'true,specified');
+});
+
 test.serial('pkg.bin as a string should work', t => {
 	meow({
 		pkg: {