[Vuejs]-How to get data in array JSON many layers

Your lessonsCourse property gets overwritten in each forEach cycle.

You can use spread operator to concatenate the section lessons array to your main array:

this.eventCourse.forEach(course => {
  course.sections.forEach(section => {
    this.lessonsCourse = [...this.lessonsCourse, ...section.lessons];
  });
});

Or, if your compiler doesn’t support spread syntax using push:

this.eventCourse.forEach(course => {
  course.sections.forEach(section => {
    section.lessons.forEach(lesson => {
      this.lessonsCourse.push(lesson);
    });
  });
});