JS: Branch Control: if then else, switch

By Xah Lee. Date: . Last updated: .

If Then Else

Simple “if” statement.

if (3 < 4) {console.log("yes");};

“if else” statement.

if (3 <= 4) {console.log("yes");}
else {console.log("no");}

“else if” chain.

const x = 3;
if (x == 1) {console.log("is 1");}
else if (x == 2) {console.log("is 2");}
else if (x == 3) {console.log("is 3");}
else {console.log("not found");}

Curly brackets {} is optional if there's just one statement, but semicolon is still required.

if (3 > 4) console.log("yes");
else console.log("no");
// no

“if then else” Expression

JavaScript also supports “if then else” EXPRESSION. (aka “ternary expression”)

test ? expr1 : expr2 → if test eval to true, then return expression expr1, else return expr2.

console.log ( 
    (4 > 5) ? "yes" : "no"
);
// no

Switch Statement

// example of switch statement

const x = "a";

switch(x) {
    case "w":
        console.log("is w");
        break; // without “break”, it'll continue to run rest without testing
    case "a":
        console.log("is a");
        break;
    case 3:
        console.log("is 3");
        break;
    default:
        console.log("none of the above");
}

switch uses === for comparison. (The === operator does not do automatic type conversion.)

JavaScript Switch is Like Goto

JavaScript's “switch” does fall-through. It'll jump to a matching point and run all of the rest of case code without testing. Think of JavaScript switch as goto.

// JavaScript's “switch” jumps to a point and continue from there.
const x = "w";

switch(x) {
    case "w":
        x += "1";
    case 32:
        x += "2";
    default:
        x += "3";
}
console.log(x); // w123

JavaScript Basics

  1. JavaScript Basics
  2. Value Types
  3. true, false
  4. Operators
  5. Variable
  6. if then else
  7. for while do Loop
  8. Array Basics
  9. Object Basics
  10. RegExp Tutorial
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