Add lesson 11 (#11)

* Add lesson 11

index.html

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lesson11.md

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+<!-- .slide: id="lesson11" -->
+
+# Basic Frontend - Spring 2024
+
+Lesson 11, Tuesday, 2024-04-23
+
+---
+
+### Recap: HTML and Javascript
+
+Basic button with click handler:
+
+```html
+<button onclick="handleButtonClick()">Click me!</button>
+```
+
+In javascript:
+
+```js
+function handleButtonClick() {
+  console.log("The button was clicked!");
+}
+```
+
+---
+
+### Recap: DOM object
+
+`document.body` is an object, we can change the (CSS) style properties of it:
+
+```js
+// change the page background color:
+document.body.style.backgroundColor = "red";
+
+// Make our click handler change the background color!
+function handleButtonClick() {
+  console.log("The button was clicked!");
+  document.body.style.backgroundColor = "green";
+}
+```
+
+---
+
+### Recap: Find HTML elements using Javascript
+
+By giving an element an `id` attribute:
+
+```html
+<div id="myDiv">Hello!</div>
+```
+
+We can access it in javascript:
+
+```js
+let someDiv = document.getElementById("myDiv");
+someDiv.textContent = "Goodbye!";
+someDiv.style.backgroundColor = "blue";
+```
+
+---
+
+### More practice with DOM and functions
+
+---
+
+### Task 1
+
+In HTML, create a div, an input, and a button.
+
+When the user puts their name into the input and clicks the button, set the div content to:
+
+`"Hello {Name}!"`
+
+---
+
+### Task 2
+
+In HTML, create an input, and a button.
+
+When the user types a color into the input and clicks the button, set the body background color to what the user input.
+
+So if the user types "blue" into the input and clicks the button, the page background color should turn blue.
+
+---
+
+### Task 3
+
+You are tasked with creating a basic calculator application that can perform addition and subtraction.
+
+Provide two input fields for operands (Operand 1 and Operand 2), a dropdown menu for the operator (+ and -), a "Calculate" button, and a div element for displaying the result.
+
+When the user clicks the "Calculate" button, retrieve the values from the input fields and the selected operator from the dropdown menu.
+
+Using JavaScript, perform the appropriate operation (addition or subtraction) based on the selected operator.
+
+Display the result in the designated div.
+
+---
+
+![calculator](images/calculator.png) <!-- .element height="300px" width="500px" -->

toc.md

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 5. [DOM](#dom)
 
 
-Direct link to lessons: [1](#lesson1) [2](#lesson2) [3](#lesson3) [4](#lesson4) [5](#lesson5) [6](#lesson6) [7](#lesson7) [8](#lesson8) [9](#lesson9) [10](#lesson10)
+Direct link to lessons: [1](#lesson1) [2](#lesson2) [3](#lesson3) [4](#lesson4) [5](#lesson5) [6](#lesson6) [7](#lesson7) [8](#lesson8) [9](#lesson9) [10](#lesson10) [11](#lesson11)
 
 NOTE: when we have too many entries that don't fit on one screen we can use this <!-- .slide: style="font-size:80%" -->