Getting to know Vue. Is this method for working with data done correctly?

Question

Vue.js

Getting to know Vue.
I am writing the toggleTask method for ToDo (i.e. marks done / not done).
Is this solution correct? Are there any Vue-way ways to improve it?

methods:{
    toggleTask(id){
      let taskIndex;
      this.tasks.forEach( (item, index) => {
        if ( item.id === id){
          taskIndex = index
        }
      });

      this.tasks[taskIndex].completed = !this.tasks[taskIndex].completed;
    }
  },

As I understand it, if immutable data operations are preferable in react, then here they are mutable.

Answer 1

You can just pass the entire task, and it all boils down to:
task.completed = !task.completed