# Get the first and last moment of a day in Javascript

Here are 2 ready-to-use functions to get the first and last millisecond of a day using a few lines of vanilla JavaScript.

## Get the first moment of a day

```
/**
 * Get first millisecond of a date
 * @param {Date} dateObj - JS Date object, default is current time
 * @returns date object with time set to first millisecond of the day
 */
const beginningOfDay = function beginningOfDay (dateObj) {
  //Get current date
  const now = dateObj || new Date();
  const year = now.getFullYear();
  const month = now.getMonth();
  const day = now.getDate();

  return new Date(year, month, day);
};
``` 

### 1. Set a default value to the date argument 
In case of no argument we set the date to current date and time. The below basically means if `dateObj` is defined set `now` to that, otherwise set it to a new `Date` object.

``` 
const now = dateObj || new Date();
``` 

### 2. Extract the date data
`new Date()` returns the current date and time. Similarly, any `Date` object you input as an argument will probably also have a time. To get the start of a day we will need a fresh start. For that we first extract the year, month and day.

``` 
const now = dateObj || new Date();
const year = now.getFullYear();
const month = now.getMonth();
const day = now.getDate();
``` 

### 3. We generate and return a "fresh" date
By generating a new `Date` using `year`, `month` and `day` as arguments you set the date of the JavaScript `Date` object. Because you didn't set the time, JS will set it to the first value which is 0 (0 hours, 0 minutes, 0 seconds, 0 milliseconds) . 

``` 
return new Date(year, month, day);
``` 

## Get the last moment of a day

```
/**
 * Get last millisecond of a date
 * @param {Date} dateObj - JS Date object, default is current time
 * @returns date object with time set to last millisecond of the day
 */
const lastMomentOfDay = function lastMomentOfDay(dateObj) {
  //Tomorrow = Current day + 1
  let tomorrow = beginningOfDay(dateObj);
  tomorrow.setDate(tomorrow.getDate() + 1);

  //Tommorow - 1 milisecond = result
  let result = new Date();
  result.setTime(tomorrow.getTime() - 1);
  return result;
};
``` 

### 1. Get the first moment of the next day
We leverage the function we wrote above (have a 🍪 for keeping your code [DRY][1]).
``` 
  let tomorrow = beginningOfDay(dateObj);
  tomorrow.setDate(tomorrow.getDate() + 1);
``` 

### 2. Subtract a millisecond from tomorrow
Remember, we got the first moment of tomorrow. If we subtract a millisecond from that, we will get the last millisecond the the day before, which is exactly what we are looking for.


[1]: https://en.wikipedia.org/wiki/Don%27t_repeat_yourself