Like Object.assign but recursive and cloning arrays, not linking them

mergeDeep.js

// ---------------------------------------------------------------------
// javascript
// Like Object.assign but recursive and cloning arrays, not linking them
// ---------------------------------------------------------------------
function mergeDeep(/* obj1, obj2, ...*/) {
	const isObject = (o)=>o && o !== undefined && o !== null && typeof o === 'object' && !Array.isArray(o) 
	let output = {}

	Object.keys(arguments).forEach( a=>{
		let o = arguments[a]
		if (!isObject(o)) throw 'mergeDeep: ERROR: all arguments must be objects'
		Object.keys(o).forEach(key=>{
			if (!isObject(o[key])) {
				if (Array.isArray(o[key])) output[key] = o[key].slice(0) 			// arrays: cloned and replaced (items not merged)
				else output[key] = o[key]																			// numbers and strings: cloned;  functions: linked
			} else {
				if (!output[key] || !isObject(output[key])) output[key] = {}	// ensure dest is object
				output[key] = mergeDeep(output[key],o[key])										// recurse
			}
		})
	})
	
	return output;
}