CODEWITHSUNDEEP
javascriptdate
Mangosteen
Mangosteen

Reputation: 191

why getDay() in javascript return +1 day of a week?

Today is 07 May 2016, Saturday But when I implement a js practice, the getDay() always return the correct number plus one. So I did this test.

enter image description here

Today is Saturday, so I expect to return 5 rather than 6.

Upvotes: 0

Views: 3174

Answers (3)

Mehmet Uzel
Mehmet Uzel

Reputation: 1

you can simply add 6 and use modulus operator.

var d = new Date(2016, 5, 20);

var weekday = (d.getDay() + 6) % 7;

Upvotes: 0

Ali Mamedov
Ali Mamedov

Reputation: 5256

The day of the week:

var d = new Date(2016, 5, 20); // June 20, 2016
d.getDay();

Starting the count from 0 (Sunday), 1 means Monday and etc...

@Mike C:

  • 0 Sunday

  • 1 Monday

  • 2 Tuesday

  • 3 Wednesday

  • 4 Thursday

  • 5 Friday

  • 6 Saturday

You will have the same situation with month too. The value returned by getMonth() is an integer between 0 and 11. 0 corresponds to January, 1 to February, and so on.

  • 0 January

  • 1 February

  • 2 March

  • 3 April

  • 4 May

  • 5 June

  • 6 July

  • 7 August

  • 8 September

  • 9 October

  • 10 November

  • 11 December

var d = new Date(2016, 5, 20); // June 20, 2016

d.getMonth(); // 5

Why?

As assumed @ChristopherW this can help to reference in an array of names. 

var 
  months = ["January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December"],
  days = ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday'];

var d = new Date(2016, 5, 20); // June 20, 2016

months[d.getMonth()]; // "June"
days[d.getDay()]; // "Monday"

It is very convenient for developers. But there is another reason for zero-indexing. The syntax of JavaScript is actually derived from C. Lets analize C's localtime function.

The C library function struct tm *localtime(const time_t *timer) uses the time pointed by timer to fill a tm structure with the values that represent the corresponding local time. The value of timer is broken up into the structure tm and expressed in the local time zone.

struct tm *localtime(const time_t *timer)

This function returns a pointer to a tm structure with the time information filled in. Following is the tm structure information:

struct tm {
   int tm_sec;         /* seconds,  range 0 to 59          */
   int tm_min;         /* minutes, range 0 to 59           */
   int tm_hour;        /* hours, range 0 to 23             */
   int tm_mday;        /* day of the month, range 1 to 31  */
   int tm_mon;         /* month, range 0 to 11             */
   int tm_year;        /* The number of years since 1900   */
   int tm_wday;        /* day of the week, range 0 to 6    */
   int tm_yday;        /* day in the year, range 0 to 365  */
   int tm_isdst;       /* daylight saving time             */ 
};

As you see month starts from 0 to 11, day of the week from 0 to 6.

Upvotes: 0

Mike Cluck
Mike Cluck

Reputation: 32511

The getDay() method returns the day of the week for the specified date according to local time, where 0 represents Sunday.

Source

So if Sunday is 0 then you get these values for the other days:

  1. Monday
  2. Tuesday
  3. Wednesday
  4. Thursday
  5. Friday
  6. Saturday

Upvotes: 3

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