index.js

'use strict';

const preserveCamelCase = string => {
	let isLastCharLower = false;
	let isLastCharUpper = false;
	let isLastLastCharUpper = false;

	for (let i = 0; i < string.length; i++) {
		const character = string[i];

		if (isLastCharLower && /[a-zA-Z]/.test(character) && character.toUpperCase() === character) {
			string = string.slice(0, i) + '-' + string.slice(i);
			isLastCharLower = false;
			isLastLastCharUpper = isLastCharUpper;
			isLastCharUpper = true;
			i++;
		} else if (isLastCharUpper && isLastLastCharUpper && /[a-zA-Z]/.test(character) && character.toLowerCase() === character) {
			string = string.slice(0, i - 1) + '-' + string.slice(i - 1);
			isLastLastCharUpper = isLastCharUpper;
			isLastCharUpper = false;
			isLastCharLower = true;
		} else {
			isLastCharLower = character.toLowerCase() === character && character.toUpperCase() !== character;
			isLastLastCharUpper = isLastCharUpper;
			isLastCharUpper = character.toUpperCase() === character && character.toLowerCase() !== character;
		}
	}

	return string;
};

const camelCase = (input, options) => {
	if (!(typeof input === 'string' || Array.isArray(input))) {
		throw new TypeError('Expected the input to be `string | string[]`');
	}

	options = Object.assign({
		pascalCase: false
	}, options);

	const postProcess = x => options.pascalCase ? x.charAt(0).toUpperCase() + x.slice(1) : x;

	if (Array.isArray(input)) {
		input = input.map(x => x.trim())
			.filter(x => x.length)
			.join('-');
	} else {
		input = input.trim();
	}

	if (input.length === 0) {
		return '';
	}

	if (input.length === 1) {
		return options.pascalCase ? input.toUpperCase() : input.toLowerCase();
	}

	const hasUpperCase = input !== input.toLowerCase();

	if (hasUpperCase) {
		input = preserveCamelCase(input);
	}

	input = input
		.replace(/^[_.\- ]+/, '')
		.toLowerCase()
		.replace(/[_.\- ]+(\w|$)/g, (_, p1) => p1.toUpperCase())
		.replace(/\d+(\w|$)/g, m => m.toUpperCase());

	return postProcess(input);
};

module.exports = camelCase;
// TODO: Remove this for the next major release
module.exports.default = camelCase;