JS Daily 35 - Automatic Semicolon Insertion

Last updated on 25 Dec, 2020

What is the output of following code?

const a = 10
const b = a
(() => console.log(b))()

The output is:

TypeError: a is not a function

If you thought the output as 10, then it is wrong. It would have been 10, if we explicitly placed semicolon at the end of each line.

const a = 10;
const b = a;
(() => console.log(b))(); // 10

When JavaScript tries to automatically insert semi-colon, it follows a process. It take each segment of the code and join them, till a valid expression is formed.

Here the code in line 1 is

const a = 10

JavaScript then tries to place next word next to 10, like

const a = 10const

Above combination is not valid according to JavaScript rules. So, it automatically inserts a semicolon after 10. Now let us see how it looks when JavaScript tries to attach line 3 at the end of line 2. It looks like this:

const b = a(() => console.log(b))()

Above statement looks valid for JS engine. It tries to execute a as a function. That is why, it results in TypeError.

--- ○ ---
Joby Joseph
Web Architect