if..else / if.. else if

if : 조건이 true인 경우 statement1 실행

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

 

if..else : 조건이 true인 경우 statement1 실행. false일 경우 statement2실행

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..else if : 조건1이 true인 경우 statement1실행. 조건1이 false이고 조건2가 true인 경우 실행

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
}

출처: W3C school(https://www.w3schools.com/js/js_if_else.asp)