execa/lib/stdio.js

'use strict';
const alias = ['stdin', 'stdout', 'stderr'];

const hasAlias = opts => alias.some(x => Boolean(opts[x]));

module.exports = opts => {
	if (!opts) {
		return null;
	}

	if (opts.stdio && hasAlias(opts)) {
		throw new Error(`It's not possible to provide \`stdio\` in combination with one of ${alias.map(x => `\`${x}\``).join(', ')}`);
	}

	if (typeof opts.stdio === 'string') {
		return opts.stdio;
	}

	const stdio = opts.stdio || [];

	if (!Array.isArray(stdio)) {
		throw new TypeError(`Expected \`stdio\` to be of type \`string\` or \`Array\`, got \`${typeof stdio}\``);
	}

	const result = [];
	const len = Math.max(stdio.length, alias.length);

	for (let i = 0; i < len; i++) {
		let value = null;

		if (stdio[i] !== undefined) {
			value = stdio[i];
		} else if (opts[alias[i]] !== undefined) {
			value = opts[alias[i]];
		}

		result[i] = value;
	}

	return result;
};