Callback binding

Maybe it would be useful to mention callback binding, I know that personally was confusing for me in the beginning. For instance, you might expect this to work:

Something.prototype.foo = function (cb) {
  someOtherObj.bar(function (err, res) {
    cb(null, this.value)
  })
}

But actually it will not work since the innermost callback is called from the context of someOtherObj, and this doesn't resolve to the Something instance (so this.value end up being something else).

The solutions are foo(function () { this.value... }.bind(this)), var self = this; foo(function () { self.value... }), or maybe ES6 arrow functions (since they take their context from wherever they were created), () => { this.value... }.

No file, test, or documentation entry point is named; first locate the section covering JavaScript callbacks and this binding. Add a clear example of the confusing context and the suggested binding alternatives, then verify the documentation renders correctly.