What does it mean when variable value in round brackets?

Question

What does it mean when variable value in round brackets in js? For example,

let a = (1,2,3);

What does it mean and why console.log(a) output is 3?

What is usage of comma operator in round brackets in variable initialization?

Answer 1

The parentheses are needed for grouping. In a let statement, commas are normally used to separate multiple variables that are being declared, e.g.

let a = 1, b = 2, c;

which is short for

let a = 1;
let b = 2;
let c;

If you write

let a = 1, 2, 3;

you'll get a syntax error, because after the comma it expects another variable declaration; it's equivalent to:

let a = 1;
let 2;
let 3;

The second and third declarations are clearly wrong, as 2 and 3 are not variable names.

The parentheses indicate that the whole expression 1, 2, 3 is being used to initialize one variable.

The expression 1, 2, 3 uses the Comma operator, which executes each of its subexpressions and returns the last one as its value. It's pretty useless when the subexpressions are all constants, so I assume your code was just a simplified example. Because the way it's written, it's really just equivalent to:

let a = 3;