JavaScript Basics: Understanding Program Structure

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Programming Structure

Now that we know about values and types, let’s talk about how to actually put them together into a running program. This is where we cover expressions, statements, variables, and the rules that make up JavaScript’s basic structure.

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Statements and Expressions

In JavaScript, a program is made up of statements. A statement might do something (like change a variable or call a function) or produce a value. Programs are just a sequence of these statements.

1;
!false;

These are statements that produce values but don’t store or use them. Usually, you’ll combine statements with variables, functions, and control flow.

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Function Calls

One of the simplest actions is to call a function:

alert("Hello world");

Here, alert is called with a string argument. Arguments are the values between the parentheses.

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Multiple Statements

We usually separate statements with semicolons. While sometimes optional, including them is a safe habit:

alert("Hello");
alert("Goodbye");

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Variables

We use variables to store values for reuse. Create them with let or const:

let message = "Hello";
alert(message);

We can change variables declared with let:

let counter = 1;
counter = counter + 1;

const means the binding can’t be changed to another value:

const pi = 3.14;

Even with const, if the value is an object, the object’s contents can still change — but the name cannot point to a new object.

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Operators

Variables can be updated with:

• = assignment
• +=, -=, *=, /= for updating in place
• ++ and -- for incrementing and decrementing by one

let number = 5;
number += 2;   // 7
number++;

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Conditional Execution – if / else

We can make code run only in certain cases using if:

let num = 10;
if (num > 5) {
  console.log("Big number!");
}

We add else for the “otherwise” branch:

if (num > 20) {
  console.log("Huge number!");
} else {
  console.log("Not that huge");
}

Or chain with else if:

if (num > 20) {
  console.log("Huge");
} else if (num > 10) {
  console.log("Large");
} else {
  console.log("Small");
}

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Switch Statements

When you need to compare a value against several possible matches, switch can be cleaner than multiple else if blocks.

let fruit = "apple";
switch(fruit) {
  case "apple":
    console.log("Apples are red or green!");
    break;
  case "banana":
    console.log("Bananas are yellow!");
    break;
  case "orange":
    console.log("Oranges are... orange!");
    break;
  default:
    console.log("I don't know that fruit.");
}

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Loops

while runs as long as its condition remains true:

let count = 0;
while (count < 3) {
  console.log(count);
  count++;
}

do...while always runs the body once before checking:

let value = 0;
do {
  console.log(value);
  value++;
} while (value < 3);

for loops are compact for counting through a range:

for (let i = 0; i < 3; i++) {
  console.log(i);
}

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Breaking and Continuing

break exits the loop early; continue skips to the next iteration:

for (let i = 0; i < 5; i++) {
  if (i === 2) continue; // skip 2
  if (i === 4) break;    // stop at 4
  console.log(i);
}

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Functions

Functions let us group code to reuse it:

function square(x) {
  return x * x;
}
console.log(square(12)); // 144

A function can have multiple parameters:

function add(a, b) {
  return a + b;
}
console.log(add(2, 3)); // 5

If return is omitted, the function returns undefined.

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Comments

Comments are notes for humans reading the code; JavaScript ignores them.

• Single-line comments start with //
• Multi-line comments go between /* and */

// This is a single-line comment
let count = 0; // comment after code

/*
This is a
multi-line comment
*/

Use comments to explain why something is done, not just what it does.

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Capitalization Conventions

JavaScript is case-sensitive — myVariable and myvariable are two different identifiers.

• Use camelCase for variables and functions → thisIsCamelCase
• Use SCREAMING_SNAKE_CASE for constants → MAX_WIDTH
• Use PascalCase for class names → Car, UserProfile

Choose a style and stick with it for readability.

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Summary

In program structure we’ve learned that:

• Programs are built from statements and expressions.
• Variables store values; operators change or compare them.
• Conditionals like if/else and switch decide what code runs.
• Loops execute code multiple times, with break and continue to adjust the flow.
• Functions package code to reuse it.
• Comments help humans understand code without changing behavior.
• Consistent capitalization keeps code clear and predictable.

With these fundamentals, you can already write small but meaningful programs in JavaScript.