3일차 공부 내용

1. Arrays

Array contents have a natural order, and keys are numeric and sequential.

Arrays in JavaScript have the following properties:

• Array size is not fixed; you can add or remove elements at any time.
• Arrays are not homogeneous; each individual element can be of any type.
• Arrays are zero-based. That is, the first element in the array is element 0.

To create an array literal in JavaScript, use square brackets, with the elements of the
array separated by commas:

const a1 = [1, 2, 3, 4]; // array containing numbers
const a2 = [1, 'two', 3, null]; // array containing mixed types
const a3 = [ // array on multiple lines
"What the hammer? What the chain?",
"In what furnace was thy brain?",
"What the anvil? What dread grasp",
"Dare its deadly terrors clasp?",
];
const a4 = [ // array containing objects
{ name: "Ruby", hardness: 9 },
{ name: "Diamond", hardness: 10 },
{ name: "Topaz", hardness: 8 },
];
const a5 = [ // array containing arrays
[1, 3, 5],
[2, 4, 6],
];

2. Dates and Times

the built-in Date object.

To create a date that’s initialized to the current date and time

const now = new Date();

To create a date that’s initialized to a specific date (at 12:00 a.m.):

const halloween = new Date(2021, 9, 31); // note that months are
// zero-based: 9=October

To create a date that’s initialized to a specific date and time:

const halloweenParty = new Date(2021, 9, 31, 19, 0); // 19:00 = 7:00 pm

3. Regular Expressions

A regular expression (or regex or regexp) is something of a sublanguage of JavaScript.

it represents a compact way to perform complex search and replace operations on strings.

Regular expressions in JavaScript are represented by the RegExp object, and they have a literal syntax consisting of symbols between a pair of forward slashes. 

// extremely simple email recognizer
const email = /\b[a-z0-9._-]+@[a-z_-]+(?:\.[a-z]+)+\b/;
// US phone number recognizer
const phone = /(:?\+1)?(:?\(\d{3}\)\s?|\d{3}[\s-]?)\d{3}[\s-]?\d{4}/;

4. Maps and Sets

ES6 introduces the data types Map and Set, and their “weak” counterparts, WeakMap and WeakSet.  

5. Data Type Conversion

5.1 Converting to Numbers

const numStr = "33.3";
const num = Number(numStr); // this creates a number value, *not*
// an instance of the Number object

If the string can’t be converted to a number, NaN will be returned.

const a = parseInt("16 volts", 10); // the " volts" is ignored, 16 is
// parsed in base 10
const b = parseInt("3a", 16); // parse hexadecimal 3a; result is 58
const c = parseFloat("15.5 kph"); // the " kph" is ignored; parseFloat
// always assumes base 10

A Date object can be converted to a number that represents the number of milliseconds since midnight, January 1, 1970, UTC, using its valueOf() method: 

const d = new Date(); // current date
const ts = d.valueOf(); // numeric value: milliseconds since
// midnight, January 1, 1970 UTC

5.2 Converting to String

All objects in JavaScript have a method toString().

const n = 33.5;
n; // 33.5 - a number
const s = n.toString();
s; // "33.5" - a string
const arr = [1, true, "hello"];
arr.toString(); // "1,true,hello"