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# Day 15/2

Plan 15/2: 

* 8h-15h: learning/practicing Js
* 15h\~ : customize speed  

## Javascript

### 1. classList

* `.classList.add('newClass')`: add a new class to the target
* `.classList.remove('newClass')`: remove a class from the target
* `.classList.contains('className')`: check if the target has a specific class

### 2. Array

Array is a special Object

`const myArray = [1, 2, 3, 'hello', {name: 'ball';}, [3, 2, 1]];`

`myArray[0] = 1;`

Cause it is an Object, it also has methods and properties

* `.length`: array 's length
* `.push()`: push an item to the end of the array
* `.splice(x, y)`: remove y items from position x

When creating an array, it creates a reference to the actual data in memory. 

`array1 = [1, 2, 3];`

`array2 = array1;`

`array3 = array1.slice();`

`array1` and `array2` point to the same data in memory, 1 array change make the other change

`array3` make a copy of the data, any change is individual 

### 3. Loops

Run the code inside as long as the condition is true. Loops create a scope.

* while loop: used with non-standard loop which means loop with no jumping step, more flexible

`while (condition) { }`

* for loop: used with standard loop which have steps

`for (let i = a; i <= b; i++) { }`