softBindingProblem.js

function foo() {
  console.log( this.a );
}

function doFoo(fn) {
  fn();
}

function doFoo2(fn, that) {
  fn.call(that);
}

var obj = {
    a: 2,
    foo: foo
};

var a = 4;

doFoo( obj.foo ); // 4
doFoo2( obj.foo, obj ); // 2