The whole point of your myNamespace property seems more than just questionable, but if you insist and still need a function that is bound to your class instance, just bind the function in the constructor, or use an arrow function which does not have its own this, but keeps this whatever it pointed to at the time of definition: (code example demonstrates both):

class Sample {
  key = '';

  constructor(key) {
    this.key = key;
    this.myNamespace.saySomething = this.myNamespace.saySomething.bind(this);
  }

  myNamespace = {
    saySomething: function(message) {
      console.log('message:', message);
      console.log('key:', this.key);
    }
  }
  
  myOtherNamespace = {
    saySomething: (message) => {
      console.log('message:', message);
      console.log('key:', this.key);
    }
  }

  getTheKey() {
    console.log('key', this.key);
  }
}

let sample = new Sample('thekey');

sample.myNamespace.saySomething('message'); // shows-> key: thekey
sample.myOtherNamespace.saySomething('other message'); // shows-> key: thekey
sample.getTheKey(); // shows-> key: thekey

"This" is pointing to two different things. "This" in your namespace function doesn't have a key value, but your constructed "sample" does - your getTheKey function accurately points to the correct "this".

To be more specific, getTheKey points to the constructed sample as "this" and the function within saySomething is pointing to saySomething as "this". The constructed sample has a value for key and saySomething does not.

You can use an arrow function instead, like so:

 myNamespace = {
    saySomething: (message) => {
      console.log('message:', message);
      console.log('key:', this.key);
    }

to target your constructed sample instead.