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JavaScript: Get ISO-8601 week number of year, weeks starting on Monday

JavaScript Datetime: Exercise-24 with Solution

Write a JavaScript function to get ISO-8601 week number of year, weeks starting on Monday.

Example : 42 (the 42nd week in the year)
Test Data :
dt = new Date(2015, 10, 1);
console.log(ISO8601_week_no(dt));
44

Sample Solution:-

HTML Code:

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>JavaScript function to get ISO-8601 week number of year, weeks starting on Monday</title>
</head>
<body>

</body>
</html>

JavaScript Code:

function ISO8601_week_no(dt) 
  {
     var tdt = new Date(dt.valueOf());
     var dayn = (dt.getDay() + 6) % 7;
     tdt.setDate(tdt.getDate() - dayn + 3);
     var firstThursday = tdt.valueOf();
     tdt.setMonth(0, 1);
     if (tdt.getDay() !== 4) 
       {
      tdt.setMonth(0, 1 + ((4 - tdt.getDay()) + 7) % 7);
        }
     return 1 + Math.ceil((firstThursday - tdt) / 604800000);
        }

dt = new Date();
console.log(ISO8601_week_no(dt));

dt = new Date(2015, 10, 1);
console.log(ISO8601_week_no(dt));

Sample Output:

25
44

Flowchart:

Flowchart: JavaScript- Get ISO-8601 week number of year, weeks starting on Monday

Live Demo:

See the Pen JavaScript - Get ISO-8601 week number of year, weeks starting on Monday-date-ex-24 by w3resource (@w3resource) on CodePen.


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Previous: Write a JavaScript function to get English ordinal suffix for the day of the month, 2 characters (st, nd, rd or th.).
Next: Write a JavaScript function to get a full textual representation of a month, such as January or June.

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JavaScript: Tips of the Day

Log the values that are commented out after the console.log statement

function* startGame() {
  const answer = yield 'Do you love JavaScript?';
  if (answer !== 'Yes') {
    return "Oh wow... Guess we're gone here";
  }
  return 'JavaScript loves you back ?';
}

const game = startGame();
console.log(/* 1 */); // Do you love JavaScript?
console.log(/* 2 */); // JavaScript loves you back ?

A generator function "pauses" its execution when it sees the yield keyword. First, we have to let the function yield the string "Do you love JavaScript?", which can be done by calling game.next().value.
Every line is executed, until it finds the first yield keyword. There is a yield keyword on the first line within the function: the execution stops with the first yield! This means that the variable answer is not defined yet!
When we call game.next("Yes").value, the previous yield is replaced with the value of the parameters passed to the next() function, "Yes" in this case. The value of the variable answer is now equal to "Yes". The condition of the if-statement returns false, and JavaScript loves you back ? gets logged.

Ref: https://bit.ly/3jFRBje