Build README

README.md

@@ -29,6 +29,7 @@
 * [Distance between two points](#distance-between-two-points)
 * [Divisible by number](#divisible-by-number)
 * [Drop elements in array](#drop-elements-in-array)
+* [Element is visible in viewport](#element-is-visible-in-viewport)
 * [Escape regular expression](#escape-regular-expression)
 * [Even or odd number](#even-or-odd-number)
 * [Factorial](#factorial)
@@ -305,6 +306,26 @@ const dropElements = (arr,func) => {
 // dropElements([1, 2, 3, 4], n => n >= 3) -> [3,4]
 ```
 
+### Element is visible in viewport
+
+Use `Element.getBoundingClientRect()` and the `window.inner(Width|Height)` values 
+to determine if a given element is visible in the viewport. 
+Omit the second argument to determine if the element is entirely visible, or specify `true` to determine if
+it is partially visible.
+
+```js
+const elementIsVisibleInViewport = (el, partiallyVisible = false) => {
+  const { top, left, bottom, right } = el.getBoundingClientRect();
+  return partiallyVisible
+    ? ((top > 0 && top < innerHeight) || (bottom > 0 && bottom < innerHeight)) &&
+      ((left > 0 && left < innerWidth) || (right > 0 && right < innerWidth))
+    : top >= 0 && left >= 0 && bottom <= innerHeight && right <= innerWidth;
+}
+// e.g. 100x100 viewport and a 10x10px element at position {top: -1, left: 0, bottom: 9, right: 10}
+// elementIsInViewport(el) -> false (not fully visible)
+// elementIsInViewport(el, true) -> true (partially visible)
+```
+
 ### Escape regular expression
 
 Use `replace()` to escape special characters.