function setup(env) {
	createDebug.debug = createDebug;
	createDebug.default = createDebug;
	createDebug.coerce = coerce;
	createDebug.disable = disable;
	createDebug.enable = enable;
	createDebug.enabled = enabled;
	createDebug.humanize = require('ms');
	createDebug.destroy = destroy;

	Object.keys(env).forEach(key => {
		createDebug[key] = env[key];
	});

	/**
	* The currently active debug mode names, and names to skip.
	*/

	createDebug.names = [];
	createDebug.skips = [];

	/**
	* Map of special "%n" handling functions, for the debug "format" argument.
	*
	* Valid key names are a single, lower or upper-case letter, i.e. "n" and "N".
	*/
	createDebug.formatters = {};

	/**
	* Selects a color for a debug namespace
	* @param {String} namespace The namespace string for the debug instance to be colored
	* @return {Number|String} An ANSI color code for the given namespace
	* @api private
	*/
	function selectColor(namespace) {
		let hash = 0;

		for (let i = 0; i < namespace.length; i++) {
			hash = ((hash << 5) - hash) + namespace.charCodeAt(i);
			hash |= 0; // Convert to 32bit integer
		}

		return createDebug.colors[Math.abs(hash) % createDebug.colors.length];
	}
	createDebug.selectColor = selectColor;

	/**
	* Create a debugger with the given `namespace`.
	*
	* @param {String} namespace
	* @return {Function}
	* @api public
	*/
	function createDebug(namespace) {
		let prevTime;
		let enableOverride = null;
		let namespacesCache;
		let enabledCache;

		function debug(...args) {
			// Disabled?
			if (!debug.enabled) {
				return;
			}

			const self = debug;

			// Set `diff` timestamp
			const curr = Number(new Date());
			const ms = curr - (prevTime || curr);
			self.diff = ms;
			self.prev = prevTime;
			self.curr = curr;
			prevTime = curr;

			args[0] = createDebug.coerce(args[0]);

			if (typeof args[0] !== 'string') {
				// Anything else let's inspect with %O
				args.unshift('%O');
			}

			// Apply any `formatters` transformations
			let index = 0;
			args[0] = args[0].replace(/%([a-zA-Z%])/g, (match, format) => {
				// If we encounter an escaped % then don't increase the array index
				if (match === '%%') {
					return '%';
				}
				index++;
				const formatter = createDebug.formatters[format];
				if (typeof formatter === 'function') {
					const val = args[index];
					match = formatter.call(self, val);

					// Now we need to remove `args[index]` since it's inlined in the `format`
					args.splice(index, 1);
					index--;
				}
				return match;
			});

			// Apply env-specific formatting (colors, etc.)
			createDebug.formatArgs.call(self, args);

			const logFn = self.log || createDebug.log;
			logFn.apply(self, args);
		}

		debug.namespace = namespace;
		debug.useColors = createDebug.useColors();
		debug.color = createDebug.selectColor(namespace);
		debug.extend = extend;
		debug.destroy = createDebug.destroy; // XXX Temporary. Will be removed in the next major release.

		Object.defineProperty(debug, 'enabled', {
			enumerable: true,
			configurable: false,
			get: () => {
				if (enableOverride !== null) {
					return enableOverride;
				}
				if (namespacesCache !== createDebug.namespaces) {
					namespacesCache = createDebug.namespaces;
					enabledCache = createDebug.enabled(namespace);
				}

				return enabledCache;
			},
			set: v => {
				enableOverride = v;
			}
		});

		// Env-specific initialization logic for debug instances
		if (typeof createDebug.init === 'function') {
			createDebug.init(debug);
		}

		return debug;
	}

	function extend(namespace, delimiter) {
		const newDebug = createDebug(this.namespace + (typeof delimiter === 'undefined' ? ':' : delimiter) + namespace);
		newDebug.log = this.log;
		return newDebug;
	}

	/**
	* Enables a debug mode by namespaces. This can include modes
	* separated by a colon and wildcards.
	*
	* @param {String} namespaces
	* @api public
	*/
	function enable(namespaces) {
		createDebug.save(namespaces);
		createDebug.namespaces = namespaces;

		createDebug.names = [];
		createDebug.skips = [];

		let i;
		const split = (typeof namespaces === 'string' ? namespaces : '').split(/[\s,]+/);
		const len = split.length;

		for (i = 0; i < len; i++) {
			if (!split[i]) {
				// ignore empty strings
				continue;
			}

			namespaces = split[i].replace(/\*/g, '.*?');

			if (namespaces[0] === '-') {
				createDebug.skips.push(new RegExp('^' + namespaces.substr(1) + '$'));
			} else {
				createDebug.names.push(new RegExp('^' + namespaces + '$'));
			}
		}
	}

	/**
	* Disable debug output.
	*
	* @return {String} namespaces
	* @api public
	*/
	function disable() {
		const namespaces = [
			...createDebug.names.map(toNamespace),
			...createDebug.skips.map(toNamespace).map(namespace => '-' + namespace)
		].join(',');
		createDebug.enable('');
		return namespaces;
	}

	/**
	* Returns true if the given mode name is enabled, false otherwise.
	*
	* @param {String} name
	* @return {Boolean}
	* @api public
	*/
	function enabled(name) {
		if (name[name.length - 1] === '*') {
			return true;
		}

		let i;
		let len;

		for (i = 0, len = createDebug.skips.length; i < len; i++) {
			if (createDebug.skips[i].test(name)) {
				return false;
			}
		}

		for (i = 0, len = createDebug.names.length; i < len; i++) {
			if (createDebug.names[i].test(name)) {
				return true;
			}
		}

		return false;
	}

	/**
	* Convert regexp to namespace
	*
	* @param {RegExp} regxep
	* @return {String} namespace
	* @api private
	*/
	function toNamespace(regexp) {
		return regexp.toString()
			.substring(2, regexp.toString().length - 2)
			.replace(/\.\*\?$/, '*');
	}

	/**
	* Coerce `val`.
	*
	* @param {Mixed} val
	* @return {Mixed}
	* @api private
	*/
	function coerce(val) {
		if (val instanceof Error) {
			return val.stack || val.message;
		}
		return val;
	}

	/**
	* XXX DO NOT USE. This is a temporary stub function.
	* XXX It WILL be removed in the next major release.
	*/
	function destroy() {
		console.warn('Instance method `debug.destroy()` is deprecated and no longer does anything. It will be removed in the next major version of `debug`.');
	}

	createDebug.enable(createDebug.load());

	return createDebug;
}
This is the common logic for both the Node.js and web browser
implementations of `debug()`.