how to assign parameter passed to constructor to local private variable with the same name

the question seems very primitive but i am new to javascript and vuejs. as shown in the code posted below,i am creating a class DigitizePolygonInteractions that extends MapBuilder. the constructor receives two parameters moduleName and mapInstance.

i want to assign the parameter mapInstance that is passed to the constructor to a local private variable named mapInstance as well. as for the code posted below, visual-studio codes does not highlight the let mapInstance when the following line of code
is added this.mapInstance = mapInstance, which indicates that this.mapInstance is not used.

in java, given the code posted below, for the statement this.mapInstance = mapInstance, this.mapInstance refers to let mapInstance while mapInstance refers to mapInstance that is passed to the constructor

how can i achieve the same in javascript please

code:

let instance;
let mapInstance //<==should be the local private variable

export class DigitizePolygonInteractions extends MapBuilder {
 /** 
 *  @param { string } moduleName like 'map-sen2bee'
 */
  constructor(moduleName,mapInstance) {
    super(moduleName);
    instance = this
    this.mapInstance = mapInstance //<==============here

}

let instance;
let mapInstance;

export class DigitizePolygonInteractions extends MapBuilder {
  constructor(moduleName, _mapInstance) {
    super(moduleName);
    instance = this;
    mapInstance = _mapInstance;
  }
}

However, a better approach would be:

export class DigitizePolygonInteractions extends MapBuilder {
  constructor(moduleName, mapInstance) {
    super(moduleName);
    this.instance = this;
    this.mapInstance = mapInstance;
  }
}

let instance;
let mapInstance // this variable will be scoped to the file or module.  


export class DigitizePolygonInteractions extends MapBuilder {
 /** 
 *  @param { string } moduleName like 'map-sen2bee'
 */
  constructor(moduleName, _mapInstance) {
    super(moduleName);
    instance = this
    this.mapInstance = _mapInstance; // this.mapInstance is a property on the object that this class will generate. 
    mapInstance  = _mapInstance; // mapInstance is a variable that exists outside this class. Basically it has nothing to do with this class.
    
   }

I haven't worked with classes that much, but I can try to explain it with a very simple example below;

let name;

class Person{
  constructor(_name, age){
    // 'this' here refers to the object that will be created when this class is instantiated (see below)
   this.name = _name;
   name = _name;
   this.age = age;  
} 
}

const boy1 = new Person('jack', 21);

console.log(boy1) // {name: 'jack', age: 21}
console.log(name) // jack 

As far as I know, the variables must be in the context of the class itself in order to access them via this. Unfortunately I am using typescript instead of pure javascript, but a working example in typescript would be:

let instance;

export class DigitizePolygonInteractions extends MapBuilder {

  private mapInstance: [Type] = {}; //<== is a local private variable of the class now and can be accessed via 'this'

  constructor(moduleName,mapInstance) {
    super(moduleName);
    instance = this
    this.mapInstance = mapInstance 
    ...