In JavaScript, you can assign multiple variables in the same line of code:

// prettier-ignore
var a = 1, b = 2;
console.log(a, b); // 1 2

And you can also assign several variables together:

// prettier-ignore
function f() {
    var a = b = 2;
}
f();
console.log(a); // ReferenceError: a is not defined
console.log(b); // 2

What just happened? Assignment is right associative (pairs from right to left), so you can think of the previous code as being the same as:

// prettier-ignore
function f() {
    var a = (window.b = 2);
    // same as var a = b = 2;
}
f();
console.log(a); // ReferenceError: a is not defined
console.log(b); // 2

The right most pair is like an assignment when you forget to put the var keyword.

You can also think about var a = (b = 3) as:

b = 3;
var a = b;

Avoid creating global variables (window.a is a global variable). If you create global variables, you can create bugs when you use the same variable name.