// This is what people normally do in ES5 to set default params
function link(height, color, callbackFn) {
  var height = height || 50;
  var color = color || 'red';
  var callbackFn = callbackFn || function() {};
​
  // function content...
}

// So there is a better way to do this, it checks param is actually undefined or not:
function link(height, color, callbackFn) {
  var height = typeof height !== 'undefined' ?  height : 50;
  var color = typeof color !== 'undefined' ?  color : 'red';
  var callbackFn = typeof callbackFn !== 'undefined' ?  callbackFn : function() {};
​
  // function content...
}

function link(height = 50, color = 'red', callbackFn = () => {}) {
  // function content...
}
​
// or using ES6 const and let
const noop = () => {};
const link = (height = 50, color = 'red', callbackFn = noop) => {
  // function content...
};