エレメントの位置を取得する

DocumentTopPosition.js

class DocumentTopPosition {
  constructor(doc, win) {
    this.document = doc || document;
    this.window = win || window;
  }

  /**
   * @returns {number} top position
   */
  get(element) {
    return this.window.pageYOffset + element.getBoundingClientRect().top;
  }

  /**
   * @param {string} selector
   * @returns {{element, top:number}} element and top position objects
   */
  find(selector) {
    const element = this.document.querySelector(selector);
    if(!element) {
      throw 'element not found'
    }
    return {element:element, top:this.get(element)};
  }
  
  /**
   * @param {string} selector
   * @returns {[{element, top:number}]} element and top position objects
   */
  findAll(selector) {
    var list = this.document.querySelectorAll(selector);
    list.map = Array.prototype.map;
    return list.map(v => ({element:v, top:this.get(v)}));
  }
}

const documentTopPosition = new DocumentTopPosition();