Close. Assigning "foo" without var will look up the lexical scope stack from most nested to global. If nothing exists, it will indeed make a new global. However, if there is a "foo" somewhere in that scope stack, it will update that variable.

    var a = function () {
      var foo
      var b = function () {
        foo = 3
      }
      b()
    }
    a()
    console.log( foo )  // undefined

In the code above, the "foo" in function a is set to 3, rather than creating a global. (changing the "vars" to "lets" would do the same thing, FWIW)

Also, "use strict" mode will not let you make a global that way. If in strict mode, you have to put the "var" outside of any function, then assign it (either on that line, or later) to make a global.