How to access variable of one class from another class?

what I do not understand too well the classes

class FirstClass {
 constructor() {
 this.importantVar = '1234asd'
}

 someFunction() {
alert(this.importantVar);
}
}

class SecondClass extends FirstClass {
 constructor(importantVar) {
super();
 this.justAnotherVar = 'ooooo';
}

 iWannaImportantVar() {
alert(this.importantVar);
}
}

gasgasgas function() {
 const firstClass = new FirstClass();
 const secondClass = new SecondClass();

firstClass.someFunction();
secondClass.iWannaImportantVar();
}

gasgasgas();