How does the comma operator work in js?

问题

I'm trying to understand how the comma operator (,) works in JavaScript, it seems to have a different behaviour when it's not put between parenthesis.

Can someone explain me why ?

Exemple for reference :

var a = 1; 
var b = 2; 
var c = (a,b);
console.log(c);
//output : as expected 
var c = a,b;
console.log(c);
//output : 1 

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[EDIT] The title might be a bit confusing. My question is about a misconception between the coma operator and var attribution as somone explained further down

Therefore this subject is not a duplicate of that one What does a comma do in JavaScript expressions?

回答1:

var c = (a,b);

The above uses the comma operator. It evaluates as the value of its right-hand side (i.e. b).

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var c = a,b;

This does not use the comma operator.

The comma character here forms part of the var expression which takes a comma-separated list of variables to create in the current scope, each of which can have an optional assignment.

It is equivalent to:

var c = a;
var b;

来源：https://stackoverflow.com/questions/50678527/how-does-the-comma-operator-work-in-js