Added alert component

snippets/Alert.md

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+---
+title: Alert
+tags: visual,beginner,state,effect
+---
+
+Creates an alert component with `type` prop.
+
+- Define appropriate CSS styles and animations for the component's elements.
+- Use the `React.setState()` hook to create the `isShown` and `isLeaving` state variables and set their values to `false`.
+- Define `timeoutId` to keep the timer instance for clearing on component unmount.
+- Use the `React.setEffect()` hook to update the value of `isShown` to `true` and clear interval by using `timeoutId` when component is unmounted.
+- Define `closeNotification` function to set the component is removed from DOM by displaying fading out animation and set `isShown` to `false` via `setTimeout()`. 
+- Define the component, which renders alert component with user defined `message` and a close button to remove the component from DOM.
+
+```css
+@keyframes leave {
+  0% { opacity: 1 }
+  100% { opacity: 0 }
+}
+
+.alert {
+  padding: 0.75rem 0.5rem;
+  margin-bottom: 0.5rem;
+  text-align: left;
+  padding-right: 40px;
+  border-radius: 4px;
+  font-size: 16px;
+  position: relative;
+}
+
+.alert.success {
+  color: #155724;
+  background-color: #d4edda;
+  border-color: #c3e6cb;
+}
+
+.alert.info{
+  color: #004085;
+  background-color: #cce5ff;
+  border-color: #b8daff;
+}
+
+.alert.warning{
+  color: #856404;
+  background-color: #fff3cd;
+  border-color: #ffeeba;
+}
+
+.alert.danger{
+  color: #721c24;
+  background-color: #f8d7da;
+  border-color: #f5c6cb;
+}
+
+.alert.leaving{
+  animation: leave 0.5s forwards;
+}
+
+.alert .close {
+  position: absolute;
+  top: 0;
+  right: 0;
+  padding: 0 0.75rem;
+  color: #333;
+  border: 0;
+  height: 100%;
+  cursor: pointer;
+  background: none;
+  font-weight: 600;
+  font-size: 16px;
+}
+```
+
+```jsx
+function Notification(props) {
+    const [isShown, setIsShown] = React.useState(false);
+    const [isLeaving, setIsLeaving] = React.useState(false);
+
+    let timeoutId = null;
+
+    React.useEffect(() => {
+      setIsShown(true);
+      return () => {
+        clearTimeout(timeoutId);
+      }
+    }, [props.isShown, props.timeout, timeoutId]);
+
+    const closeNotification = () => {
+      setIsLeaving(true);
+      timeoutId = setTimeout(() => {
+        setIsLeaving(false);
+        setIsShown(false);
+      }, 250)
+    }
+
+    return isShown && (
+      <div className={`alert ${props.type}${isLeaving ? ' leaving' : ''}`}>
+        <button className="close" onClick={closeNotification}>x</button>
+        {props.message}
+      </div>
+    )
+}
+```
+
+```jsx
+ReactDOM.render(<Notification type="info" message="This is info" />, document.getElementById('root'));
+```