Learn Javascript -2

• 	if, else if, and else statements

• 	comparison operators

• 	logical operators

• 	truthy vs falsy values

• 	ternary operators

• 	switch statement

[if]
In programming, we can also perform a task based on a condition using an if statement:

구조 :
if (true) {
console.log('This message will print!');
}
괄호(condition) + 코드 블럭{}

Let 과 var의 차이는 뭘까?
Let 은 true와 false값 지정이 가능한 것?

[comparison operators]

• 	Less than: <

• 	Greater than: >

• 	Less than or equal to: <=

• 	Greater than or equal to: >=

• 	Is equal to: ===

• 	Is not equal to: !==

let hungerLevel = 7;
if (hungerLevel > 7 )
{
console.log('Time to eat!');
}
else {
console.log('We can eat later!');
}

&& operator

Boolean = true, false
불리언 값과 함께 쓰이는 오퍼레이터
There are three logical operators:

• 	the and operator (&&)

• 	the or operator (||)

• 	the not operator, otherwise known as the bang operator (!)

if (stopLight === 'green' && pedestrians === 0) {
console.log('Go!');
} else {
console.log('Stop');
}

if (day === 'Saturday' || day === 'Sunday') {
console.log('Enjoy the weekend!');
} else {
console.log('Do some work.');
}

let excited = true;
console.log(!excited); // Prints false

let sleepy = false;
console.log(!sleepy); // Prints true

let myVariable = 'I Exist!';

Check if the variable has been assigned a value.

if (myVariable) {
console.log(myVariable)
} else {
console.log('The variable does not exist.')
}

<—!false value—>

• 	0

• 	Empty strings like "" or ''

• 	null which represent when there is no value at all

• 	undefined which represent when a declared variable lacks a value

• 	NaN, or Not a Number

let numberOfApples = 0;

if (numberOfApples){
console.log('Let us eat apples!');
} else {
console.log('No apples left!');
}

// Prints 'No apples left!'

[short-circuit evaluation - 단락 평가]
let tool = '';

// Use short circuit evaluation to assign writingUtensil variable below:
let writingUtensil = tool || 'pen'

console.log(The ${writingUtensil} is mightier than the sword.);

->

let tool = 'marker';

// Use short circuit evaluation to assign writingUtensil variable below:
let writingUtensil = tool || 'pen'

console.log(The ${writingUtensil} is mightier than the sword.);

[tenary operator]

let isNightTime = true;

if (isNightTime) {
console.log('Turn on the lights!');
} else {
console.log('Turn off the lights!');
}

isNightTime ? console.log('Turn on the lights!') : console.log('Turn off the lights!');

Ex2)

const plantNeedsWater = day => 
day === 'Wednesday' ? true : false;

const plantNeedsWater = day => 
day === 'Wednesday' ? true : false;

[else if statement]

//예제
let stopLight = 'yellow';

if (stopLight === 'red') {
console.log('Stop!');
} else if (stopLight === 'yellow') {
console.log('Slow down.');
} else if (stopLight === 'green') {
console.log('Go!');
} else {
console.log('Caution, unknown!');
}

A [switch statement] provides an alternative syntax that is easier to read and write. A switch statement looks like this:

구조 :
Switch (변수) {
Condition-
Case ‘value’ :
Console.log(‘…’);
Vreak;
Case ‘value’ :
Console.log(‘…’);
qksqhr.
Default:
Console.log(‘Invalid item’);
Break;
}

//예제

let groceryItem = 'papaya';

switch (groceryItem) {
case 'tomato':
console.log('Tomatoes are $0.49');
break;
case 'lime':
console.log('Limes are $1.49');
break;
case 'papaya':
console.log('Papayas are $1.29');
break;
default:
console.log('Invalid item');
break;
}

// Prints 'Papayas are $1.29'