Optional Semicolon in JavaScript

Published on 5 Feb, 2021

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;

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.


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


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?


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.