class Foo {

    constructor() {
        this.func = null;
    }

    a() {
      let self = this;
      this.func = function() {
        console.log(self);
        console.log(this);
      }
      let testFunc = function() {
        console.log('in test func')
        console.log(this);
      }
      testFunc();
    }

    b() {
      console.log('in b')
      this.func();
    }
}

let x = new Foo();
x.a();
x.b();