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187 lines
3.0 KiB
187 lines
3.0 KiB
# Intro to JS
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## Lesson Objectives
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1. Add comments to code
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1. Describe the basic data types of JS
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1. Assign values to variables
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1. Concatenate values
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1. Boolean expressions
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1. Operators
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1. Write a While loop
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1. Write a For loop
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## Add comments to code
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single line comment:
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```
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// this is a single line comment
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```
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multi-line comment:
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```
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/*
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this is
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a mult-line
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comment
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*/
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```
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## Describe the basic data types of JS
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Strings:
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```javascript
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console.log("hello world");
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```
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Numbers:
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```javascript
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console.log(100);
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```
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## Assign values to variables
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```javascript
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var phrase = 'In my room is a chair and a table';
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console.log(phrase);
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```
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**We will not be using `var`** (it's a bit out of date), instead we will be using **`let`** and **`const`**.
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Let's use `const` here:
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```javascript
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const phrase = 'In my room is a chair and a table';
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const sum = 99 + 1;
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```
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`const` variables are **constant** and do not change. Now that the phrase is saved to a variable, we can call it whenever.
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```javascript
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console.log(phrase);
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console.log(sum);
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```
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### Variable re-assignment
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With `const`, you cannot reassign a variable. It is CONSTANT.
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```javascript
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const item = ' chair';
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item = 'eclair';
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```
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You can re-assign variables with `let`:
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```javascript
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let item = 'chair';
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item = 'eclair';
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console.log(item);
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```
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## Concatenate values
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JavaScript allows us to add strings together with `+`:
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```javascript
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console.log('hello' + ' world');
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```
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We can insert values of variables into our strings:
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```javascript
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const adjective = 'beautiful';
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console.log('What a ' + adjective + ' day!!');
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```
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## Boolean expressions
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Test various kinds of equality
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* `>` greater than
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* `<` less than
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* `==` equal to
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* `>=` greater than or equal to
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* `<=` less than or equal to
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* `!=` is not equal
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```javascript
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console.log(5 <= 10);
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console.log("asdf" != "fun");
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```
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## Operators
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### Math
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We can do arithmetic with numbers:
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```javascript
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console.log(100 + 100);
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console.log(100 - 50);
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console.log(100 * 10);
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console.log(100 / 10);
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```
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#### Postfix operator
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```javascript
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i++;
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i--;
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```
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is the same as
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```javascript
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i = i + 1;
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i = i - 1;
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```
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#### Compound assignment operator `+=`
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```javascript
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i += 3; //increments by 3
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i -= 3; //decrement by 3
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```
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is the same as
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```javascript
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i = i + 3;
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i = i - 3;
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```
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## Write a While loop
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```javascript
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let num = 1;
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while (num <= 1000) {
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console.log('The number is: ' + num);
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num++;
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}
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```
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## Write a For loop
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```javascript
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for (let i=1; i <= 1000; i++) {
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console.log('The number is: ' + i);
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}
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```
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There are three parts to the 'control panel' of the loop, delimited by the semicolon:
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```javascript
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for (initial condition; while condition; repeating expression) {
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}
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```
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1. some initial code supplied to the loop -- usually a numerical starting value of the loop
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2. the condition under which the loop runs -- it will run while the expression is true
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3. a repeating expression that runs at the end of each loop -- usually an instruction to increase the numerical starting value
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