Ever wonder how Javascript interprets your code under the hood?

Let's zoom out and see what the interpreter sees — and why it matters.

When we write Javascript, it's easy to think of it like a list of commands:

let name = "Alex";
console.log(name);

But Javascript doesn't just run these commands mindlessly.

It parses, evaluates, and sometimes... "fixes" our mistakes 🤖

That's where concepts like statements, expressions, comments, and the infamous semicolon debate come in.

Let's break it all down — including some surprising behaviour from something called Automatic Semicolon Insertion (ASI).

🧱 Statements vs Expressions — What's the Difference?

Understanding the distinction between statements and expressions is not just important, it's foundational.

Everything in Javascript, from the simplest to the most complex, is either a statement or an expression. This knowledge will empower you to write more efficient and effective code.

✅ A Statement:

A command. It does something.

let age = 30;

if (age > 18) {
  console.log("Adult");
}

let, if, for, return — these are statements.

🧠 Think of statements as sentences. They execute an action.

✅ An Expression:

A unit of code that produces a value.

5 + 5; // 10
"Hi" + "JS"; // "HiJS"
Math.max(2, 4); // 4

Even this is an expression:

let sum = 2 + 3;

Here, 2 + 3 is an expression. The full line is a statement.

🧠 If it returns something, it's an expression. If it just does something, it's likely a statement.

🔁 Expressions Inside Statements

Often, we use expressions inside statements:

if (user.age > 18) { // expression: user.age > 18
  console.log("Welcome");
}

So remember:

• An expression becomes part of a larger statement

• Expressions return values; statements perform actions

💬 Comments — Talking to Yourself (and Others)

Javascript lets us leave notes in our code using comments — and we should!

✅ Single-Line Comments:

// This is a one-line comment
let score = 10; // You can also put it after the code

✅ Multi-Line Comments:

/*
This is a block comment.
It can span multiple lines.
Useful for documentation!
*/

Use comments to:

• Explain why you're doing something (not just what)

• Leave reminders (// TODO: clean this up)

• Temporarily disable the code

🧠 Good comments are gold.

❓ Semicolons: Optional, Required, or What?

Here's the thing: Javascript lets us skip semicolons, but should we?

let x = 5
let y = 10
console.log(x + y)

This works. But here's the twist: it works because Javascript inserts the semicolons for us... most of the time.

Let's unpack that.

🧠 Automatic Semicolon Insertion (ASI) — Javascript Fixes Your Code?

Javascript has a rule-based feature called ASI — Automatic Semicolon Insertion.

When the interpreter reads your code, if it sees a line break where it expects a semicolon, it'll often insert one for you.

Great, right?

Not always.

❗ Classic ASI Gotcha:

return
{
  message: "Hello"
}

We think this returns an object.

Javascript sees it as:

return; // <-- ASI inserts semicolon here
{ message: "Hello" } // This becomes a block, not an object

So it returns undefined, not the object we expected 😬

✅ When ASI Works Fine:

• Simple variable assignments

• Console logs

• Function calls

But...

❌ When ASI Breaks Stuff:

• return statements on a new line

• IIFEs (Immediately Invoked Function Expressions)

• ++ or -- in specific line-break situations

📏 Should We Use Semicolons?

Some teams say, "Skip them, the interpreter handles it."

Others say, "Always use them for safety."

Here's our take:

✔️ Use them. It avoids edge cases, clarifies intent, and saves debugging time.

It's one character. We'll live 😄

⚠️ Real Gotchas We Should Know

Let's look at a few more subtle issues where the way Javascript parses code can trip us up.

⛔ ASI Misleading Errors:

let b = "ASI Gotcha!"
const a = b
(1 + 2).toString() // TypeError: b is not a function

Javascript might read this as a function call on b, not on 1 + 2.

Running it in console,

Adding a semicolon solves it:

let b = "ASI Gotcha!"
const a = b;
(1 + 2).toString(); // '3'

📚 Reference Specs (For the Curious)

If we want to go full deep-dive mode, check out:

• MDN: Statements and Declarations

• ECMA-262 Specification — the official Javascript language spec

This is where the language rules are defined. Hardcore, but worth knowing exists.

🧠 Recap — What We've Learned

• Statements perform actions; Expressions return values

• Comments help us explain intent, document behaviour, and improve readability

• Semicolons are mostly optional, but ASI can misinterpret code, especially with return

• Use semicolons consistently to avoid bugs and make intent clear

• Javascript does a lot of interpretation — but it's not magic. It follows strict parsing rules.

🔗 Next Up:

👉 Time to talk about Variables, Declarations and headaches 😏