Control Flow: if/else, switch, and Loops

Control flow determines the order in which statements are executed. By using conditionals and loops, you can make your programs react to different inputs and repeat tasks efficiently.

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if / else if / else

The most fundamental conditional statement:

const temperature = 28;

if (temperature > 30) {
  console.log("It's hot!");
} else if (temperature > 20) {
  console.log("It's warm.");
} else if (temperature > 10) {
  console.log("It's cool.");
} else {
  console.log("It's cold!");
}
// Output: "It's warm."

Rules

• The condition inside if() is converted to a boolean.
• Only the first branch whose condition is truthy will execute.
• else if and else are optional.

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Truthy and Falsy Values

JavaScript converts any value to a boolean when needed. Values that convert to false are called falsy:

Falsy Values
false
0, -0, 0n
"" (empty string)
null
undefined
NaN

Everything else is truthy — including "0", "false", empty arrays [], and empty objects {}.

if ("hello") {
  console.log("Truthy!");   // this runs
}