Control Flow: if, else, and switch

Control flow determines which code runs and in what order. JavaScript provides if/else statements and switch statements to branch your program based on conditions.

The if Statement

let temperature = 28;

if (temperature > 30) {
  console.log("It's hot outside!");
}

The block inside if only runs when the condition evaluates to true. Curly braces define the block — always use them even for single-line bodies.

if / else

let score = 72;

if (score >= 90) {
  console.log("A grade");
} else if (score >= 80) {
  console.log("B grade");
} else if (score >= 70) {
  console.log("C grade");
} else {
  console.log("Below C");
}

Only one branch executes — the first one whose condition is true. Once a branch is taken, the rest are skipped entirely.

Truthy and Falsy Values

JavaScript evaluates any value as either truthy or falsy in a boolean context. Falsy values are:

false
0
""          // empty string
null
undefined
NaN

Everything else is truthy, including "0", [], and {}.

let username = "";

if (username) {
  console.log("Welcome, " + username);
} else {
  console.log("Please enter a username");  // this runs
}

The switch Statement

Use switch when comparing a single value against many specific cases:

let day = "Monday";

switch (day) {
  case "Monday":
  case "Tuesday":
  case "Wednesday":
  case "Thursday":
  case "Friday":
    console.log("Weekday");
    break;
  case "Saturday":
  case "Sunday":
    console.log("Weekend");
    break;
  default:
    console.log("Unknown day");
}

The break statement exits the switch block. Without it, execution "falls through" to the next case, which can be used intentionally (as shown above with weekdays) but is often a bug.

Logical Operators in Conditions

let age = 25;
let hasTicket = true;

if (age >= 18 && hasTicket) {
  console.log("Entry granted");
}

let isAdmin = false;
let isModerator = true;

if (isAdmin || isModerator) {
  console.log("Has elevated access");
}

Guard Clauses

A common pattern is to return early from a function to avoid deep nesting:

function processOrder(order) {
  if (!order) {
    return "No order provided";
  }
  if (!order.isPaid) {
    return "Payment required";
  }
  // main logic here — no deep nesting needed
  return "Order processed";
}

Mastering if/else and switch is fundamental. Use guard clauses to keep code readable, and always use strict equality in conditions.