MVC structure

[maurusrv]

constructor(board, tiles, pieces) {
this.promotionPrompt = document.querySelector(
      ".promotion-prompt-container"
    );
    this.promotionPromptBlack = document.querySelector(
      ".promotion-prompt-black"
    );
    this.promotionPromptWhite = document.querySelector(
      ".promotion-prompt-white"
    );
    this.piecePromotionButton = document.querySelectorAll("[promotion-button]");
 ...

document calls should not be inside a class. this further improves MVC, separates concerns. anyway, you can pass it also as a parameter for the constructor like what you did in app.js