Develop Reference

How does valueOf() method works on a date object in JavaScript underneath?

Given a date :
const date = new Date();
console.log(date.valueOf()); // print numeric value : 1587644687189
Any documentation mentions that valueOf() is an inbuilt function in JavaScript which is used to get the number of milliseconds between 1 January 1970 00:00:00 UTC and the given date.
How this function is implemented on a Date object ?

Date instances are always based on a timestamp value, a number that measures milliseconds from a fixed point in time. The .valueOf() function simply returns that number. The function does not have to compute the number of milliseconds; it always has that as effectively a private internal property.
Calling the various mutator functions like .setMonth() etc will update that timestamp value. That process is where interesting work is done, such as taking into account daylight savings time and other complexities of calendar math.
Any object can have a .valueOf():
console.log(5 + { valueOf: () => 6 }); // 11
It can also be placed on the prototype, as is the case with Date objects.

Get today's day from Date.now()

After getting the value from Date.now() I have tried extracting the date like that:
Date.now().getDate()
However, the getDate() function doesn't work on it. My whole application is based on dates that are retrieved from Date.now() and I need to get current day of the month from the Date.now() value. I have done some research and I can not find a solution to it.

Date.now() doesn't returns a Date instance:
The Date.now() method returns the number of milliseconds elapsed since
January 1, 1970 00:00:00 UTC
Being a number, it doesn't have .getDate() method. But, you can make use of Date without arguments to get a Date instance which has a .getDate() method:
If no arguments are provided, the constructor creates a JavaScript
Date object for the current date and time according to system
settings.
So this should work:
new Date().getDate(); // 3

Create a Date Object with the year only

I'm used to create Date objects by using the fourth syntax from MDN as new Date(year, month, day, hours, minutes, seconds, milliseconds); But lately I tried to set a Date object with only a year (as new Date(2017)) but as you could expect it was treated as a value and considered the year as a number of milliseconds.
Is there any way of still easily use the year as is without changing the syntax and expect a correctly set Date ?

Two solutions come to my mind:
(1) Set the year argument to 2017 and set the month argument to 0 when constructing the date:
let d = new Date(2017, 0);
console.log(d.toString());
The arguments will be treated as local time; month and day of month will be January 1; all time components will be set to 0.
(2) Specify "2017T00:00" as the first and only argument when constructing the date:
let d = new Date("2017T00:00");
console.log(d.toString());
According to current specs this is a valid format and browsers are supposed to treat it as local time. The behavior is same as that of previous example.

If you are passing a single parameter (number or string), then it is taken as per doc
value
Integer value representing the number of milliseconds since January 1,
1970 00:00:00 UTC, with leap seconds ignored (Unix Epoch; but consider
that most Unix time stamp functions count in seconds).
dateString
String value representing a date. The string should be in a format
recognized by the Date.parse() method (IETF-compliant RFC 2822
timestamps and also a version of ISO8601).
Also as per doc
If at least two arguments are supplied, missing arguments are either
set to 1 (if day is missing) or 0 for all others.
You can pass one more parameter as 0 or null (or the actual value you want to set)
new Date(2017,0);
Demo
var date = new Date(2017,0);
console.log( date );

You could pass null as second argument:
new Date(2017, null);
However, without knowing the details of how missing values are interpreted, what do you think happens now? Will the date be initialized with the current month, day, hour, etc? Or something different?
Better be explicit and pass all arguments, so that you know what the code is doing half a year later.

I have another suggestion. You can just create a regular date object and set it's year. So at least you know to expect what the rest of the Date object values are.
var year = "2014";
var date = new Date();
date.setFullYear(year);
// console.log(year) => Wed Dec 27 2014 16:25:28 GMT+0200
Further reading - Date.prototype.setFullYear()

Determine if string is Date or Number

I'm trying to determine if a string is a number or a date.
Here is my code:
this._getFieldFormat = (value) => {
// check if is date
let d = new Date(value);
if (!isNaN( d.getTime() ) ) {
return 'date';
}
// check if is boolean
// isNaN(false) = false, false is a number (0), true is a number (1)
if(typeof value === 'boolean'){
return 'boolean';
}
// check if a string is a number
if(!isNaN(value)){
return 'number';
}
return typeof value;
};
It works for a date like: 2016-04-19T23:09:10.208092Z.
The problem is that 1 look to be a valid date (Wed Dec 31 1969 16:00:00 GMT-0800 (PST)) and isNaN(new Date()) is return false (a date is a number).
Any idea on how to get out of this loop?

So what is happening is called coercion. Since javascript is dynamic typing when you give it two different types the js engine tries to coerce one of the types into something reasonable or what it thought you meant.
For instance:
isNan("37"); is false because "37" is converted to the number 37
isNaN(new Date()) is return false (a date is a number)
It converted Date to a number so this is correct.
However, invalid values in date strings not recognized as ISO format as defined by ECMA-262 may or may not result in NaN, depending on the browser and values provided
So
new Date('23/25/2014'); // NON-ISO string with invalid date values
So this will return NaN in all browsers that comply with ES5 and later.
Also to do a stricter check you can use:
Number.isNan(new Date()); // This should return true
So to recap make sure the date conform to the ISO standard or it will be NaN and use the stricter check. Hope this helps.

In general and from a Javascript design point of view, however, I don't think you can do it by design. Any number between 8640000000000000 and the earliest date in terms of a number -8640000000000000 can be converted to date (represented as time in milliseconds from 01 January, 1970 UTC).
Therefore, any number not falling is this range cannot be a date. And any number falling in range would be a valid date or a number and you gotta use context to determine if you want to interpret it as a number or a date.
You could however do a naive implementation to test if number is a valid date or not, but that's the best you can do, without context.
Determining whether a date is a number or not can be a bit easier. Because a human-readable representation of date will return true by isNaN() thereby determining that it's definitely not a number. Then you would go on and check if that string is a date or not, which you already did in your function.

Javascript Date() object with NULL Time

Is there a way to indicate only the date portion of a Date() object, without indicating the time?
e.g.
var d = Date();
d.setFullYear(2015, 0, 13);
d.toString();
"Tue Jan 13 2015 00:00:00 GMT-0500 (EST)" // Wrong - I didn't set a time!
"Tue Jan 13 2015 NULL GMT-0500 (EST)" // Expected Result
I want to be able to tell the difference between a user who only inputed the Date portion, vs one who explicitly inputed both a Date and a Time

Not really. A Javascript Date object always has a time. You can leave it at midnight and ignore it if you want, but it'll still be there. It's up to you how you interpret it.
If you want to be able to represent a null time, you could interpret midnight to mean that, though then you would have no way to represent times that actually are midnight. If you want to be able to have a null time and still represent every possible time you would need to have two variables.
You could have:
// Date with null time
var date = new Date(2015, 0, 13); // time component ignored
var time = null;
// Date with non-null time
var date = new Date(2015, 0, 13); // time component ignored
var time = new Date(1970, 0, 1, 9, 30); // date component ignored
Note in the second example the year, month and day in the time component are arbitrary and won't be used, but they still need to be there if you want to create a Date object.

JavaScript Date objects are internally defined using number of milliseconds since January 1, 1970 UTC. Therefore you are stuck with the time part.
Try this code
var date = new Date(2015, 0, 13);
console.log(date.valueOf());
You'll get this output
1421125200000
Here is the standard definition...
ECMAScript Language Spec See page 165
From ECMA standard:
A Date object contains a Number indicating a particular instant in time to within a millisecond. Such a Number
is called a time value. A time value may also be NaN, indicating that the Date object does not represent a
specific instant of time.
Time is measured in ECMAScript in milliseconds since 01 January, 1970 UTC. In time values leap seconds
are ignored. It is assumed that there are exactly 86,400,000 milliseconds per day. ECMAScript Number values
can represent all integers from –9,007,199,254,740,992 to 9,007,199,254,740,992; this range suffices to
measure times to millisecond precision for any instant that is within approximately 285,616 years, either
forward or backward, from 01 January, 1970 UTC.
The actual range of times supported by ECMAScript Date objects is slightly smaller: exactly –100,000,000
days to 100,000,000 days measured relative to midnight at the beginning of 01 January, 1970 UTC. This gives
a range of 8,640,000,000,000,000 milliseconds to either side of 01 January, 1970 UTC.
The exact moment of midnight at the beginning of 01 January, 1970 UTC is represented by the value +0

Dates are objects. As such, you can add properties to them as needed:
var date_only = new Date("2015-03-04");
date_only.time_is_meaningful = false;
var date_with_time = new Date("2015-03-04T10:47");
date_with_time.time_is_meaningful = true;
This is simpler and cleaner than your millisecond hack, and more
convenient than having two separate variables. You could then, if you
wish, subclass Date with a custom toString that checks the
time_is_meaningful property and acts accordingly.

You cannot remove the time information from a Date.
If you want to have both informations independently, use a Date for the date and ignore the time (e.g. ensure it's always exactly midnight for instance), and use another field to hold the time (it could be a Date where you ignore the date but not the time, or it could be one input text with formatted time, or several input texts with hour, minute, etc).
The UI representation is up to you.