🐾 동등 비교 및 동일성 총정리

🔎 Comparison

📌 == Operator

equal to

📌 === Operator

equal value and equal type

📌 != Operator

not equal

📌 !== Operator

not equal value or not equal type

📌 > Operator

greater than

📌 < Operator

less than

📌 >= Operator

greater than or equal to

📌 <= Operator

less than or equal to

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🔎 Logical Operators

📌 && / AND

The && operator returns true if both expressions are true, otherwise it returns false.

• 둘다성립되어야 true

let x = 6;
let y = 3; 

(x < 10 && y > 1) is true

📌 || / OR

The || returns true if one or both expressions are true, otherwise it returns false.

• 둘중에 하나라도 성립되면 true
• 둘다 성립되지 않으면 false

let x = 6;
let y = 3;
 
(x == 5 || y == 5) is false

📌 ! / NOT

The NOT operator (!) returns true for false statements and false for true statements.

!(x == y) is true
//x와 y는 같이않다. =>true

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🐾 조건문 총정리

🔎 if, else, and else if, switch

---

🔎 if Statement

📌 Use if to specify a block of code to be executed, if a specified condition is true

syntax

if (condition) {
  //  block of code to be executed if the condition is true
}

🐾 Make a "Good day" greeting if the hour is less than 18:00:

if (hour < 18) {
  greeting = "Good day";
}

---

🔎 The else Statement

📌 Use else to specify a block of code to be executed, if the same condition is false

Use the else statement to specify a block of code to be executed if the condition is false.
syntax

if (condition) {
  //  block of code to be executed if the condition is true
} else {
  //  block of code to be executed if the condition is false
}

🐾 If the hour is less than 18, create a "Good day" greeting, otherwise "Good evening":

if (hour < 18) {
  greeting = "Good day";
} else {
  greeting = "Good evening";
}

---

🔎 The else if Statement

📌 Use else if to specify a new condition to test, if the first condition is false

Use the else if statement to specify a new condition if the first condition is false.

Syntax

if (condition1) {
  //  block of code to be executed if condition1 is true
} else if (condition2) {
  //  block of code to be executed if the condition1 is false and condition2 is true
} else {
  //  block of code to be executed if the condition1 is false and condition2 is false
}

🐾 If time is less than 10:00, create a "Good morning" greeting, if not, but time is less than 20:00, create a "Good day" greeting, otherwise a "Good evening":

if (time < 10) {
  greeting = "Good morning";
} else if (time < 20) {
  greeting = "Good day";
} else {
  greeting = "Good evening";
}

---

🔎조건(삼항)연산자

condition? exprIfTrue : exprIfFalse

---

🔎 switch

📌 Use switch to specify many alternative blocks of code to be executed

The switch statement is used to perform different actions based on different conditions.

Syntax

switch(expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}

The getDay() method returns the weekday as a number between 0 and 6.
(Sunday=0, Monday=1, Tuesday=2 ..)
This example uses the weekday number to calculate the weekday name:

switch (new Date().getDay()) {
  case 0:
    day = "Sunday";
    break;
  case 1:
    day = "Monday";
    break;
  case 2:
     day = "Tuesday";
    break;
  case 3:
    day = "Wednesday";
    break;
  case 4:
    day = "Thursday";
    break;
  case 5:
    day = "Friday";
    break;
  case 6:
    day = "Saturday";
}

let a = 2;

switch(a) {
    case 1:
        console.log('a는 1입니다.');
        break;
    case 2:
        console.log('a는 2입니다.'); // a는 2입니다.
        break;
    default:
        console.log('a는 1도 2도 아닙니다.');
}

---

🐾 JS 반복문 총정리

🔎 For Loop

Instead of writing:

text += cars[0] + "<br>";
text += cars[1] + "<br>";
text += cars[2] + "<br>";
text += cars[3] + "<br>";
text += cars[4] + "<br>";
text += cars[5] + "<br>";

You can write:

for (let i = 0; i < cars.length; i++) {
  text += cars[i] + "<br>";
}

//BMW
//Volvo
//Saab
//Ford
//Fiat
//Audi

syntax

		초기값				조건식 		증감식
for ([initialization]; [condition]; [final-expression])
   statement

Expression 1 sets a variable before the loop starts (let i = 0). 

• loop이 시작하기전에 첫번째로 체크

Expression 2 defines the condition for the loop to run (i must be less than 5).

• condition 체크, 충족한지

Expression 3 increases a value (i++) each time the code block in the loop has been executed.

• loop이 실행된다음에 값이 올라감 (either puls or minus depends )
• 다음번 condition 평가 이전에 발생

---

📌 for in Loop 객체, 배열 가능

🔎 For In Loop (obj사용)

for in을 객체로 돌렸을떄.

The JavaScript for in statement loops through the properties of an Object:
📌 for in문은 object 에 사용할 수 있는 반복문, 배열에도 사용할 수 있지만 배열 반복에는 추천되지 않는다.

Syntax

for (key in object) {
  // code block to be executed
}

// 객체일때 **** => key 값 (프로퍼티명)
for (let key in obj) {
  console.log(key) // name, age
  console.log(obj[key]) // 이름, 나이
}

const person = {fname:"John", lname:"Doe", age:25};

let text = "";
for (let x in person) {
  text += x;  // fname lnam eage
  text += person[x];// John Doe 25
}

---

const obj = {
  name : '이름',
  age : '나이'
}

for(const key in obj){
  console.log(`key 값 : ${key}`); // 1. key값 : name // 2. key값 :age
  console.log(`value 값 : ${obj[key]}`); // 1. value 값 : 이름 // 2. value값 : 나이
}

obj.name과 같이 사용하려면 object내에 name이라는 key값을 가진 value가 존재해야한다.
없는 key값을 사용하게 되면 undefined가 출력

const person = {fname:"John", lname:"Doe", age:25};

let text = "";
for (let x in person) {
  //x=fname,lname,age
  text += person[x];
  //John Doe 25
}

---

🔎 For In Loop (Arrays사용)

for in을 배열로 돌렸을떄.

The JavaScript for in statement can also loop over the properties of an Array:
(키값하고 value값 모두 가능)

// 배열일때
for (let index in numbers) {
  console.log(index) // 0, 1, 2, 3, 4
  console.log(numbers[index]) // 45, 4, 9, 16, 25
}

// const numbers = {
//     0: 45,
//     1: 4,
//     2: 9,
//     …
// }

---

The for in statement can loops over array values:
(values값만 받을떄.)

const numbers = [45, 4, 9, 16, 25];

let txt = "";
for (let x in numbers) {
  txt += numbers[x];
}
//45
//4
//9
//16
//25

📌 Do not use for in over an Array if the index order is important.

---

📌 for of Loop 배열, 문자열 가능 ( 객체일때 => 불가능)

🔎 The For Of Loop

for of문은 반복 가능한 객체(Array, Map, Set, String, TypedArray, arguments 객체 등 포함)에 대해 사용할 수 있다. 보통은 Array(배열)에 사용.

Syntax

for (variable of iterable) {
  // code block to be executed
}

📌 Looping over an Array (배열)

// 배열일때 ****
for (let value of numbers) {
  console.log(value) // 45, 4, 9, 16, 25
}

---

📌 Looping over a String (문자열)

let language = "JavaScript";

let text = "";
for (let x of language) {
text += x;
}

---

🔎 forEach()

배열에 사용되는 메서드. 인자에 콜백함수를 넣어 사용

Note that the function takes 3 arguments:
The item value
The item index(생략가능)
The array itself(생략가능)

const numbers = [45, 4, 9, 16, 25];


numbers.forEach((value, index) => {
    console.log(value, index) // [45, 0], [4, 1], [9, 2], [16, 3], [25, 4]
})

numbers.forEach((value) => {
    console.log(value) // 45, 4, 9, 16, 25
})

---

🔎 While Loop

Syntax

while (condition) {
  // code block to be executed
}

condition(조건)이 truthy 이면 반복문 본문의 코드가 실행됩니다.

아래 반복문은 조건 i < 3을 만족할 동안 i를 출력해줍니다.

let i = 3;
while (i < 3) { 
  console.log( i );// 출력값없음
  i++;
}

---

🔎 Do While Loop

Syntax

do {
  // code block to be executed
}
while (condition);

let i = 3;
do {
  console.log( i );//출력값: 3
  i++;
} while (i < 3);

3을출력하고, while문으로

do..while 문법은 조건이 truthy 인지 아닌지에 상관없이, 본문을 최소한 한 번이라도 실행하고 싶을 때만 사용해야 합니다. 대다수의 상황에선 do..while보다 while(…) {…}이 적합합니다.