You know when you write a paragraph and forget to put a full stop (.) at the end of some sentences? Your teacher might still understand what you mean… but sometimes, the meaning changes, or they get confused.

In JavaScript, something similar happens with semicolons (;). Semicolons are like full stops for code—they tell the JavaScript engine: “Hey, this line is complete. Move to the next one.”

But guess what? JavaScript is kind. It says: “If you forget the semicolon, I’ll try to guess where it should be!” This is called Automatic Semicolon Insertion (ASI).

What is ASI?

ASI is a feature in JavaScript where the interpreter automatically adds semicolons to your code when you forget them.

For example:

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

There are no semicolons here, but it still works. Why? Because JavaScript secretly turns it into:

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

Cool, right? But wait… it’s not always perfect.

When ASI Works Fine

If each statement is on its own line, JavaScript can guess correctly. For example:

let x = 10
let y = 20
console.log(x + y)

JavaScript interprets this as:

let x = 10;
let y = 20;
console.log(x + y);

No problem here.

When ASI Causes Problems

Sometimes, forgetting semicolons can break your code or change its meaning. Here’s a famous example:

let a = b + c
(d + e).print()

You might think this means:

a = b + c
(d + e).print()

But JavaScript thinks:

a = b + c(d + e).print()

Why? Because it didn’t add a semicolon before (d + e).print(). It assumed that (d + e) is part of the first line!

Another Big Problem – return

Look at this code:

function getName() {
  return
  "Alex"
}
console.log(getName())

What do you think will print? "Alex"? No! It prints undefined. Why? Because JavaScript inserts a semicolon after return like this:

return;
"Alex";

So the function ends right there and never returns "Alex".

The Rule to Remember

  • ASI is helpful but dangerous.
  • Always put semicolons where they belong.
  • Don’t rely on ASI, especially after return, break, or continue.

Pro Tip

Many developers skip semicolons, and modern tools (like Prettier) format your code properly. But for beginners, always use semicolons to avoid confusion.

✅ Summary in Simple Words

  • ASI = JavaScript adds missing semicolons automatically.
  • Works most of the time, but can cause bugs in some cases.

  • Always put semicolons after:

    • return
    • break
    • continue
    • And at the end of statements.

10 Fill-in-the-Gap Questions

  1. Semicolons in JavaScript are like ______ in English sentences.
  2. ASI stands for Automatic ______ Insertion.
  3. JavaScript adds semicolons when you ______ them.
  4. return followed by a newline can cause JavaScript to insert a ______ too early.
  5. In let a = b + c (d + e).print(), ASI can cause ______ errors.
  6. ASI is helpful, but sometimes ______.
  7. If each statement is on its own ______, ASI usually works fine.
  8. Many developers use tools like ______ to format code.
  9. Always add semicolons after return, ______, and continue.
  10. ASI is a feature in ______ language.
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