Subject exercise 7 "Action - reaction!" (JS interaction / click event listener)

subjects/action-reaction/README.md

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+### Action - reaction!
+
+OK, you have now connected HTML, CSS and JS altogether ; big day! Excited? Exhausted?
+Well so far, you've only scratched the surface... Let's go deeper into the power of JS! You're going to add some interaction ; the webpage will react when a user action will happen, called an [event](https://developer.mozilla.org/en-US/docs/Web/Events) (a click, a key pressed, a mouse move, etc.).
+
+Let's put a button on the top right corner of the page, that will toggle (close or open) the left eye when clicked.
+Add it in the HTML structure:
+
+```
+<button>Click me to toggle left eye</button>
+```
+
+Add the style in the CSS file:
+
+```
+button {
+ position: fixed;
+ top: 30px;
+ right: 30px;
+ padding: 30px;
+}
+```
+
+In the JS file, get the HTML button element with [`querySelector`](https://developer.mozilla.org/en-US/docs/Web/API/Document/querySelector), and [add an event listener](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener) on [`click` event](https://developer.mozilla.org/en-US/docs/Web/API/Element/click_event#javascript), triggering a function that will:
+
+- change the [text content](https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent) of the button: if the eye is open, write "Click to close the left eye", if the eye is closed, write "Click to open the left eye"
+- [toggle](https://css-tricks.com/snippets/javascript/the-classlist-api/) the class `eye-open` in the `classList` of the `eye-left` HTML element
+- change the background color of the `eye-left`: if the eye is open, to "orange", if the eye is closed, to "black"
+
+### Notions
+
+- [`querySelector`](https://developer.mozilla.org/en-US/docs/Web/API/Document/querySelector)
+- [Text content of a HTML element](https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent)
+- [`addEventListener`](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener) / [`click` event](https://developer.mozilla.org/en-US/docs/Web/API/Element/click_event#javascript)
+- [`classList` / `toggle`](https://css-tricks.com/snippets/javascript/the-classlist-api/)
+- [Setting style with JS](https://developer.mozilla.org/en-US/docs/Web/API/ElementCSSInlineStyle/style#setting_styles)