Add observeMutations.md

snippets/observeMutations.md

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+### observeMutations
+
+Runs the provided callback for each mutation on the specified element.
+
+Use a [`MutationObserver`](https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver) to observe mutations on the given element.
+Use `Array.forEach()` to run the callback for each mutation that is observed.
+Omit the third argument, `options`, to use the default [options](https://developer.mozilla.org/en-US/docs/Web/API/MutationObserver#MutationObserverInit) (all `true`).
+
+```js
+const observeMutations = (element, callback, options) => new MutationObserver(mutations => mutations.forEach(m => callback(m))).observe(element, Object.assign(
+  {
+    childList: true,
+    attributes: true,
+    attributeOldValue: true,
+    characterData: true,
+    characterDataOldValue: true,
+    subtree: true
+  }, options));
+```
+
+```js
+observeMutations(document, console.log); // Logs all mutations that happen on the page
+```