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:

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.
Improve this sample solution and post your code through Disqus
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
Groups the elements of an array based on the given function and returns the count of elements in each group
Example:
const tips_countBy = (arr, fn) => arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => { acc[val] = (acc[val] || 0) + 1; return acc; }, {}); console.log(tips_countBy([2.5, 3.2, 4.5], Math.floor)); console.log(tips_countBy(['one', 'two', 'three'], 'length'));
Output:
[object Object] { 2: 1, 3: 1, 4: 1 } [object Object] { 3: 2, 5: 1 }
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