Add yesterday snippets

snippets/yesterday.md

@@ -0,0 +1,18 @@
+### yesterday
+
+Results in a string representation of yesterday's date.
+
+Use `new Date()` to get the current date, decrement by one using `Date.getDate()` and set the value to the result using `Date.setDate()`.
+Use `Date.prototype.toISOString()` to return a string in `yyyy-mm-dd` format.
+
+```js
+const yesterday = () => {
+  let t = new Date();
+  t.setDate(t.getDate() - 1);
+  return t.toISOString().split('T')[0];
+};
+```
+
+```js
+yesterday(); // 2018-10-17 (if current date is 2018-10-18)
+```