Optional Semicolon in JavaScript

JavaScript uses semi-colon(;) to separate two statements.

var a;
let b;

In JavaScript, if we omit the semi-colon, JavaScript intelligently understands end of statements.

var a = 6;
console.log(a); // 6

In the above code JavaScript parser first takes var and then check if adding next phrase makes any sense. var a is a valid statement. The engine does not stop there. It will check if the next part can also be appended. var a = makes sense only if there is a value or expression after =. We have 6 which is valid value. Now, the statement looks like var a = 6. Again, JavaScript checks if appending next line makes sense. var a = 6 console.log(a) is not a valid statement. So JavaScript, automatically inserts a semi-colon after 6. So our code after parsing looks like:

var a = 6;
console.log(a);

It is always good to explicitly put semicolon. Consider the following code.

var a = b(x + y).toString();

If we tried to insert 2 statements, that wont work. JavaScript engine consider above code like below.

var a = b(x + y).toString();

return, break and continue

There are 2 exceptions to the automatic semicolon insertion by JavaScript.

The first exception involves return, break and continue. Let us consider following code.

return;
true;

The second line can be considered as continuation of first line. But JavaScript does not do that. It places a semicolon immediately after return.

return;
true;

So if we want to write the return value in separate lines, place a ( as a starting point in the same line of return. That makes JavaScript to parse the full () block and return it.

return a + b;

This line break issue is with break and continue also;

Unary increment and decrement operators

The second exception to auto semicolon insertion is with unary increment and decrement operators.

How the code below works?

x;
++y;

It is parsed as x; ++y;, not as x++; y;. Anyways, I have not seen any developer writing unary operators in separate lines. Still, it is fun to know this weird behaviour of JavaScript.