Аналог функций slideUp, slideDown and SlideToggle на чистом JS

document.addEventListener("DOMContentLoaded", function() {
    'use strict';

    Element.prototype.slideUp = function (duration = 500) {
        this.style.transitionProperty = 'height, margin, padding';
        this.style.transitionDuration = duration + 'ms';
        this.style.boxSizing = 'border-box';
        this.style.height = this.offsetHeight + 'px';
        this.offsetHeight;
        this.style.overflow = 'hidden';
        this.style.height = 0;
        this.style.paddingTop = 0;
        this.style.paddingBottom = 0;
        this.style.marginTop = 0;
        this.style.marginBottom = 0;
        window.setTimeout( () => {
              this.style.display = 'none';
              this.style.removeProperty('height');
              this.style.removeProperty('padding-top');
              this.style.removeProperty('padding-bottom');
              this.style.removeProperty('margin-top');
              this.style.removeProperty('margin-bottom');
              this.style.removeProperty('overflow');
              this.style.removeProperty('transition-duration');
              this.style.removeProperty('transition-property');
        }, duration);
    }

    /* SLIDE DOWN */
    Element.prototype.slideDown = function (duration = 500) {
        this.style.removeProperty('display');
        let display = window.getComputedStyle(this).display;
        if (display === 'none') display = 'block';
        this.style.display = display;
        let height = this.offsetHeight;
        this.style.overflow = 'hidden';
        this.style.height = 0;
        this.style.paddingTop = 0;
        this.style.paddingBottom = 0;
        this.style.marginTop = 0;
        this.style.marginBottom = 0;
        this.offsetHeight;
        this.style.boxSizing = 'border-box';
        this.style.transitionProperty = "height, margin, padding";
        this.style.transitionDuration = duration + 'ms';
        this.style.height = height + 'px';
        this.style.removeProperty('padding-top');
        this.style.removeProperty('padding-bottom');
        this.style.removeProperty('margin-top');
        this.style.removeProperty('margin-bottom');
        window.setTimeout( () => {
          this.style.removeProperty('height');
          this.style.removeProperty('overflow');
          this.style.removeProperty('transition-duration');
          this.style.removeProperty('transition-property');
        }, duration);
    }

    /* TOOGLE */
    Element.prototype.slideToggle = function (duration = 500) {
        if (window.getComputedStyle(this).display === 'none') {
          return this.slideDown(duration);
        } else {
          return this.slideUp(duration);
        }
    }
});

el.slideUp();
el.slideDown();
el.slideToggle();

Источник: https://dev.to/bmsvieira/vanilla-js-slidedown-up-4dkn