如何从另一个方法中获取变量的值？(vue.js 2)

data() {
  return {
    message: 'Hello Vue!'
  }
},
methods: {
  showMessage() {
    console.log(this.message);
  }
}

data() {
  return {
    firstName: 'John',
    lastName: 'Doe'
  }
},
computed: {
  fullName() {
    return this.firstName + ' ' + this.lastName;
  }
},
methods: {
  showMessage() {
    console.log(this.fullName);
  }
}

data() {
  return {
    message: ''
  }
},
methods: {
  setMessage(value) {
    this.message = value;
  },
  showMessage() {
    this.setMessage('Hello Vue!');
    console.log(this.message);
  }
}