Custom jquery

class ElementCollection extends Array {
  ready(cb) {
    const isReady = this.some(e => {
      return e.readyState != null && e.readyState != "loading";
    });
    if (isReady) {
      cb();
    } else {
      this.on("DOMContentLoaded", cb);
    }
    return this;
  }

  on(event, cbOrSelector, cb) {
    if (typeof cbOrSelector === "function") {
      this.forEach(e => e.addEventListener(event, cbOrSelector));
    } else {
      this.forEach(elem => {
        elem.addEventListener(event, e => {
          if (e.target.matches(cbOrSelector)) cb(e);
        });
      });
    }
    return this;
  }

  next() {
    return this.map(e => e.nextElementSibling).filter(e => e != null);
  }

  prev() {
    return this.map(e => e.previousElementSibling).filter(e => e != null);
  }

  removeClass(className) {
    this.forEach(e => e.classList.remove(className));
    return this;
  }

  addClass(className) {
    this.forEach(e => e.classList.add(className));
    return this;
  }

  css(property, value) {
    const camelProp = property.replace(/(-[a-z])/, g => {
      return g.replace("-", "").toUpperCase();
    });
    this.forEach(e => (e.style[camelProp] = value));
    return this;
  }
}

class AjaxPromise {
  constructor(promise) {
    this.promise = promise;
  }

  done(cb) {
    this.promise = this.promise.then(data => {
      cb(data);
      return data;
    });
    return this;
  }

  fail(cb) {
    this.promise = this.promise.catch(cb);
    return this;
  }

  always(cb) {
    this.promise = this.promise.finally(cb);
    return this;
  }
}

function $(param) {
  if (typeof param === "string" || param instanceof String) {
    return new ElementCollection(...document.querySelectorAll(param));
  } else {
    return new ElementCollection(param);
  }
}

$.get = function ({ url, data = {}, success = () => {}, dataType }) {
  const queryString = Object.entries(data)
    .map(([key, value]) => {
      return `${key}=${value}`;
    })
    .join("&");

  return new AjaxPromise(
    fetch(`${url}?${queryString}`, {
      method: "GET",
      headers: {
        "Content-Type": dataType,
      },
    })
      .then(res => {
        if (res.ok) {
          return res.json();
        } else {
          throw new Error(res.status);
        }
      })
      .then(data => {
        success(data);
        return data;
      })
  );
};