javascriptvariable-declarationvariable-initialization

Can a variable declaration refer to other variables declared on the same line?

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?

Solution

  • 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.