Things became more complicated with the new class and Config system introduced with Ext 4 (CoreMedia upgraded directly to Ext 6, later to 7). Configs now can be declared for an Ext class and then trigger some magic: For every Config property foo, Ext generates methods getFoo() and setFoo(value). Note that these are not accessors, but "normal" methods, as Ext 4 came out when browser support for accessors was not yet mainstream. They never managed to update their Config system to "real" accessors.

To make things "easier" (?) for the developer, things get even more complicated: the generated setFoo(value) method looks for two optional "hook" methods that allow the following:

Neglecting this additional "convenience", this is how you could define a Config text, prevent anything that is not a string from being set into that Config (at least not when everybody uses the setText(value) method), and update the DOM of your component when the text is changed afterwards:

 Ext.define("acme.Label", {
  extend: "Ext.Component",
  xtype: "acme.label",
  config: {
    text: ""
  },
  setText(value) {
    this.value = typeof value === "string" ? value : value ? String(value) : "";
    // if rendered, update my DOM node with 'value'
  }
};

var label = Ext.create({ xtype: "acme.label", text: "Hi!"});
label.setText(null);
console.log(button.getText()); // logs the empty string (""), not "null"