operator, if, for loop ~(dreamcoding) | ES6

[operator]

1. String concatenation

console.log('my' + ' cat');	//my cat
console.log(`string literals: 1 + 2 = ${1+2}`); //string literals: 1 + 2 = 3

* '' 랑 `` 헷갈리지 말기 (backtick기호.)

 

2. Numeric operators

console.log(1 + 1); // add
console.log(1 - 1); // substract
console.log(1 / 1); // divide
console.log(1 * 1); // multiply
console.log(5 % 2); // remainder
console.log(2 ** 3); // exponentiation

 

3. Increment and decrement operators

let counter = 2;
const preIncrement = ++counter;
console.log(`preIncrement : ${preIncrement}, counter: ${counter}`);
//preIncrement : 3, counter: 3

const postIncrement = counter++;
console.log(`postIncrement : ${postIncrement}, counter: ${counter}`);
//postIncrement : 3, counter: 4

decrement도 같은 원리

 

4. Assignment oprators

let x = 3;
let y = 6;
x += y; //x = x + y; 
x -= y;
x *= y;
x /= y;

 

5. Comparison oprators

console.log(10 < 6); //f
console.log(10 <= 6); //f
console.log(10 > 6); //t
console.log(10 >= 6); //t

 

6. Logical operators : || (or), && (and), ! (not)

const value1 = true;
const value2 = 4 < 2;

// || (or)
//finds the first truthy value 하나라도 true 나오면 거기서 멈춤. 뒷부분 코드 아예 안읽는다.
//=> 무거운 함수일수록 뒤로 보내는게 훨씬 효율적! (check함수가 가장 뒤로오는게 효율적)
console.log(`or: ${value1 || value2 || check()}`);

// && (and)
//finds the first falsy value 하나라도 false가 나오면 거기서 멈춤.
console.log(`and: ${value1 && value2 && check()}`);

//null check할때도 많이 쓴다. // nullableObject && nullableObject.something 
if (nullableObject != null) {
 nullableObject.something;
}

function check() {
  for(let i = 0; i < 10; i++) {
    console.log('haha');
  }
  return true;
};

// ! (not)
console.log(!value1);

 

7. Equality

//7. Equality
const stringFive = '5';
const numberFive = 5;

//loose equality, with type conversion
console.log(stringFive == numberFive); //true
console.log(stringFive != numberFive); //false

//strict equality, no type conversion
console.log(stringFive === numberFive); //false
console.log(stringFive !== numberFive); //true

//object equality by reference
const ellie1 = {name: 'ellie'};
const ellie2 = {name: 'ellie'};
const ellie3 = ellie1;
console.log(ellie1 == ellie2); //false object는 참조를 가지는데 참조가 다르므로 false
console.log(ellie1 === ellie2); //false
console.log(ellie1 === ellie3); //true ellie1과 ellie3는 같은 참조(reference)를 가진다.

* equality - puzzler
console.log(0 == false); //t
console.log(0 === false); //f
console.log('' == false); //t
console.log('' === false); //f
console.log(null == undefined); //t
console.log(null === undefined); //f

 

8. Conditional operators: if

//if, else if, else
const name = 'coder';
if (name === 'ellie') {
  console.log('Welcome, Ellie!');
} else if (name === 'coder') {
  console.log('You are amazing coder');
} else {
  console.log('unknown');
}

 

9. Ternary operator: ?

//condition ? value1 : value2;
console.log(name === 'ellie' ? 'yes' : 'no');
//간단한 경우에는 이렇게 쓰지만 여러가지 쓸 경우엔 if else나 switch를 쓰는게 가독성에 좋음

 

10. Switch statement

//use for multiple if chesckx
//use for enum-like value check
//use for multiple type checks in TS
const browser = 'IE';
switch (browser) {
  case 'IE':
    console.log('go away!');
    break;
  case 'Chrome':
    console.log('love you');
    break;
  case 'Firefox':
    console.log('love you');
    break;
  default:
    console.log('same all!');
    break;
}

*switch문에서 중복된 것을 출력하는 case가 있다면 같이 쓰는것도 가능.
case 'Chrome':
case 'Firefox':
  console.log('love you!');
  break;

 

11. Loops 반복문

*while loop
//while the condition is truthy, body code is executed.

let i =3;
while (i > 0) {
  console.log(`while: ${i}`);
  i--;
}

//while: 3
//while: 2
//while: 1


*do while loop
//body code is executed first, the check the condition. 일단 실행하고 조건본다.
do {
  console.log(`do while: ${i}`);
  i--;
} while (i > 0);

// do while: 0  만 출력. 이미 i가 위에서 0.


*for loop 
//for(begin; condition; step)
for (i = 3; i > 0; i--) {
  console.log(`for: ${i}`);
}

for (let i = 3; i > 0; i = i- 2) {
  //inline variable declaration
  console.log(`inline variable for: ${i}`);
}

//nested loops
for ( let  i = 0; i < 10; i++) {
  for (let j = 0; j < 10; j++) {
    console.log(`i: ${i}, j: ${j}`);
  }
}
//이런식으로 nested해서 쓸 수 있지만 cpu에 좋지는 않음.

 

* break, continue quiz

//Q1. iterate from 0 to 10 and print only even numbers(use continue)
for(let i = 0; i < 11; i++) {
  if(i % 2 !== 0) {
    continue;
  }
  console.log(`q1. ${i}`);
}

//Q2. iterate from 0 to 10 and print numbers until reaching 8
for(let i = 0; i < 11; i++) {
  if(i > 8) {
    break;
  }
  console.log(`q2. ${i}`);  
}

* lable도 있지만 현업에서는 쓰지않음.