Control Flow: Conditionals

Control flow determines the order in which statements are executed in a script. JavaScript provides several structures for branching execution based on the evaluation of expressions.

1. The if...else Statement

The most fundamental control structure. It executes a block of code if a specified condition is Truthy.

const score = 85;

if (score >= 90) {
    console.log("Grade: A");
} else if (score >= 80) {
    console.log("Grade: B");
} else {
    console.log("Grade: C");
}

2. Truthiness and Falsiness

In JavaScript, every value has an inherent boolean state. A value is Falsy if it is one of the following eight:

1. false
2. 0 (and -0, 0n)
3. "" (Empty string)
4. null
5. undefined
6. NaN

Everything else is Truthy, including empty arrays [] and empty objects {}. This is a common point of confusion for developers coming from languages like Python.

3. The switch Statement

Used for multiple branches based on a single value. It performs a Strict Equality (===) check.

const role = "admin";

switch (role) {
    case "admin":
        initAdminDashboard();
        break; // Crucial to prevent "fall-through"
    case "editor":
        initEditor();
        break;
    default:
        initGuest();
}

4. Logical Assignment (ES2021)

Modern JavaScript provides concise operators that combine logical checks with assignment.

• ||=: Assigns if the variable is falsy.
• &&=: Assigns if the variable is truthy.
• ??=: Assigns if the variable is nullish (null or undefined).

Interview Pro-Tips: Switch vs. Object Maps

While switch is fine, senior developers often replace complex conditional logic with Object Maps or Lookup Tables for better readability and performance.

// Instead of switch:
const roleActions = {
    admin: () => initAdmin(),
    editor: () => initEditor(),
    guest: () => initGuest()
};

const action = roleActions[role] || roleActions.guest;
action();

Technical Summary

1. Branching: Evaluating expressions to decide the next path of execution.
2. Comparison: Always use strict operators inside conditionals to avoid coercion bugs.
3. Early Return: In functions, prefer returning early rather than nesting multiple if/else blocks.