🕤 dateDiff() - returns a detail object about the difference between two dates

dateDiff.ts

/**
 * ☃ dateDiff "Snowman Carl" (http://stackoverflow.com/questions/13903897)
 * Returns a detail object about the difference between two dates
 *
 * When providing custom units, provide them in descending order (eg week,day,hour; not hour,day,week)
 *
 * @param {Date} dateStart - date to compare to
 * @param {Date|string} [dateEnd=new Date()] - second date, can be used as unit param instead
 * @param {...string} [units=Object.keys(dateDiffDef)] - limits the returned object to provided keys
 */
export function dateDiff(
	dateStart:	Date,
	dateEnd:	Date | string = new Date, 
	...units:	string[]
): {
	[key: string]: number
} {
	if(typeof dateEnd === 'string')
		dateEnd = new Date();
	
	let delta: number = Math.abs(dateStart.getTime() - dateEnd.getTime());
	
	return (units.length ? units : Object.keys(dateDiffDef))
		.reduce((res: object, key: string) => {
			if(!dateDiffDef.hasOwnProperty(key))
				throw new Error('Unknown unit in dateDiff: '+key);
			res[key] = Math.floor(delta / dateDiffDef[key]);
			delta -= res[key] * dateDiffDef[key];
			return res;
		}, {});
}

// default time units for dateDiff
export const dateDiffDef = {
	millennium:	31536000000000,
	century:	3153600000000,
	decade:		315360000000,
	year:		31536000000,
	quarter:	7776000000,
	month:		2592000000,
	week:		604800000,
	day:		86400000,
	hour:		3600000,
	minute:		60000,
	second:		1000,
	millisecond:	1
};

@perdanafm you must provide a start and and end date for a valid date range, im not sure why do you mention that the starting date could be defaulted to the current date.

In fact, if you are comparing the current date and a fixed date (i.e curr and a furute date) you must add the two for generating a valid range.

The only thing i could mention for fixing, is adding an exception for cases where the delta is a negative number and/or some of the dates are null or undefined.