Getting an invalid left hand side assignment error

class Student {
    constructor(gpa, credits){
        this.gpa = gpa;
        this.credits = credits;
    }

    stringGPA() {
        return this.gpa.toString();
    }
  get major(){
  return this._major
  }
  set major(declare){
     if(this.level='Senior' || this.level= 'Junior'){
        this._major= declare }else if(this.level='Freshman' || this.level='Sophomore'){
        this._major= 'None'
        }

  }

    get level() {
        if (this.credits > 90 ) {
            return 'Senior';
        } else if (this.credits > 60) {
            return 'Junior';
        } else if (this.credits > 30) {
            return 'Sophomore';
        } else {
            return 'Freshman';
        }
    }
}

var student = new Student(3.9, 60);