Question

I was thinking about declaring vars in JS, when a doubt arose. What’s the priority of setting up two vars’ value at the same time? See:

var a = 1,
b = 1;

With this code, a is declared before b. But what about this code

var a = b = 1;

Which one does receive the value first? a or b?

Answer 1

It is evaluated as

var a = (b = 1);

Hence, b gets the value first.

Note that this differs from the first snippet: If not already in global scope, b will become a global variable (if it was not declared before). This would cause an error in strict mode.

A formal description can be found in the specification.