Add weekOfYear

snippets/weekOfYear.md

@@ -0,0 +1,25 @@
+---
+title: weekOfYear
+tags: date,advanced
+firstSeen: 2021-08-15T05:00:00-04:00
+---
+
+Returns the zero-indexed week of the year that a date corresponds to.
+
+- Use `new Date()` and `Date.prototype.getFullYear()` to get the first day of the year as a `Date` object.
+- Use `Date.prototype.setDate()`, `Date.prototype.getDate()` and `Date.prototype.getDay()` along with the modulo (`%`) operator to get the first Monday of the year.
+- Subtract the first Monday of the year from the given `date` and divide with the number of milliseconds in a week.
+- Use `Math.round()` to get the zero-indexed week of the year corresponding to the given `date`.
+- `-0` is returned if the given `date` is before the first Monday of the year.
+
+```js
+const weekOfYear = date => {
+  const startOfYear = new Date(date.getFullYear(), 0, 1);
+  startOfYear.setDate(startOfYear.getDate() + (startOfYear.getDay() % 7));
+  return Math.round((date - startOfYear) / (7 * 24 * 3600 * 1000));
+};
+```
+
+```js
+weekOfYear(new Date('2021-06-18')); // 23
+```