Question

I use this syntax often but I don’t understand why it simply works.

var a = 1, b = a + 1;
console.log(b); // 2

In the event you’re declaring a var, after splitting them on a comma, the b already sees a as evaluated? Why is that?

Answer 1

It has been defined as such.

First, the definition of the essential terms:

VariableStatement :

var VariableDeclarationList ;

and:

VariableDeclarationList :

VariableDeclaration

VariableDeclarationList , VariableDeclaration

The actual definition which applies to your question is this one:

The production VariableDeclarationList : VariableDeclarationList , VariableDeclaration is evaluated as follows:

1. Evaluate VariableDeclarationList.

2. Evaluate VariableDeclaration.

So at the time #2 is executed, #1 has been evaluated already and you can use any effect of it in #2.