2일차 공부 내용 2

1. ES6(aka Harmony, aka Java‐Script 2015, aka ECMAScript2015)

ES6 is standard rule of javascript.

ES6 code has to be transcompiled into "safe" ES5 to ensure that it can run anywhere.

There are a lot of new features in ES6.

 

2. Using Git and terminal or bash

2.1 git install

2.2 create root project repository

Type the following on the project directory

$git init

2.3 create gitignore file

Inevitably, there will be some files you never want to track in version control.

These files can be explicitly excluded in a file called gitignore.

2.4 check current status

$git status

2.5 commit 

$git add -A

$git commit -m "<brief description of the changes your just made">

 

3. Primitive Types

• Number
• String
• Boolean
• Null
• Undefined
• Symbol

 

4. Numbers

No matter what literal format you use (decimal, hexadecimal, exponential,
etc.), the number that gets created is stored in the same format:
a double.

 

5. Strings

Strings in JavaScript represent Unicode text.

In JavaScript, string literals are represented with single quotes, double quotes, or
backticks(Also called a grave accent mark).

The backtick was introduced in ES6 to enable template strings.

 

6. Escaping

const dialog1 = "He looked up and said \"don't do that!\" to Max.";
const dialog2 = 'He looked up and said "don\'t do that!" to Max.';
const s = "In JavaScript, use \\ as an escape character in strings.";

 

7. Template Strings(aka string interpolation)

let currentTemp = 19.5;
const message = `The current temperature is ${currentTemp}\u00b0C`;

ES6 introduces string templates.

 

8. Multiline Strings

const multiline = "line1\n\
line2";

const multiline = `line1
line2`;

const multiline = "line1\n" +
"line2\n" +
"line3";

const multiline = 'Current temperature:\n' +
`\t${currentTemp}\u00b0C\n` +
"Don't worry...the heat is on!";

 

9. Booleans

Booleans are value types that have only two possible values: true and false.

let heating = true;
let cooling = false;

 

10. Symbols

New in ES6 are symbols: a new data type representing unique tokens.

Symbols are created with the Symbol() constructor.

You can optionally provide a description, which is just for convenience

const RED = Symbol();
const ORANGE = Symbol("The color of a sunset!");
RED === ORANGE // false: every symbol is unique

 

11. null and undefined

null has only one possible value (null).

undefined has only one possible value (undefined).

Something hasn’t been given a value yet.

Both null and undefined represent something that doesn’t exist.

Note that if you declare a variable without explicitly giving it a value, it will have a value of undefined by default.

let currentTemp; // implicit value of undefined
const targetTemp = null; // target temp null -- "not yet known"
currentTemp = 19.5; // currentTemp now has value
currentTemp = undefined; // currentTemp appears as if it had never
// been initialized; not recommended

 

12. Objects

objects can represent multiple or complex values, and can change over their lifetime.

An object is a container, and the contents of that container can change over time.

 

The contents of an object are called properties (or members), and properties consist of a name (or key) and value. Property names must be strings or symbols, and values can be any type (including other objects).

const obj = {};
obj.size; // undefined
obj.color; // "yellow"

To use the member access operator, the property name must be a valid identifier. If you want property names that are not valid identifiers, you have to use the computed member access operator (you can also use this for valid identifiers):

obj["not an identifier"] = 3;
obj["not an identifier"]; // 3
obj["color"]; // "yellow"

You also use the computed member access operator for symbol properties.

const SIZE = Symbol();
obj[SIZE] = 8;
obj[SIZE]; // 8

obj has pointed to the same object all along, but the object itself has changed.

const sam1 = {
name: 'Sam',
age: 4,
};

const sam2 = { name: 'Sam', age: 4 }; // declaration on one line

const sam3 = {
name: 'Sam',
classification: { // property values can
kingdom: 'Anamalia', // be objects themselves
phylum: 'Chordata',
class: 'Mamalia',
order: 'Carnivoria',
family: 'Felidae',
subfaimily: 'Felinae',
genus: 'Felis',
species: 'catus',
},
};

Objects can also contain functions.

sam3.speak = function() { return "Meow!"; };

we can delete a property from an object with the delete operator:

delete sam3.classification; // the whole classification tree is removed
delete sam3.speak; // the speak function is removed

 

13. Number, String, and Boolean Objects

What JavaScript is doing is creating a temporary String object.

As soon as the function has been called, JavaScript discards the object.