textareaを中身に応じて自動リサイズする ref: https://qiita.com/sainu__/items/4b3dab630688e6a3803f

ResizeTextarea.js

/*
 * How to use
 *
 * import ResizeTextarea from './path/to/ResizeTextarea';
 * let textarea = document.querySelector('#textarea');
 * const resizeTextarea = new ResizeTextarea(textarea);
 * textarea.addEventListener('input', () => {
 *   resizeTextarea.resize();
 * });
 */

export default class ResizeTextarea {
  constructor(textarea) {
    this.textarea = textarea;
    this.style = window.getComputedStyle(textarea, null)
  }

  resize() {
    if (this.scrollHeight > this.offsetHeight) {
      this.height = this.scrollHeight;
    } else {
      while(true) {
        this.height = this.height - this.lineHeight;
        if (this.scrollHeight >= this.offsetHeight) {
          this.height = this.scrollHeight;
          break;
        }
      }
    }
  }

  // get height of textarea from inline style attribute or css
  // return Number
  get height() {
    let heightCss = this.textarea.style.height || this.style.getPropertyValue('height');
    return Number(heightCss.slice(0, -2)) || 0;
  }

  // get line-height of textarea from inline style attribute or css
  // return Number
  get lineHeight() {
    let lineHeightCss = this.textarea.style.lineHeight || this.style.getPropertyValue('line-height');
    return Number(lineHeightCss.slice(0, -2)) || 0;
  }

  // return Number
  get scrollHeight() {
    return this.textarea.scrollHeight;
  }

  // return Number
  get offsetHeight() {
    return this.textarea.offsetHeight;
  }

  // param Number
  set height(height) {
    if (typeof(height) !== 'number') console.error('expect: ResizeTextarea get heightの引数は数値です。got: ', typeof(height));
    this.textarea.style.height = height + 'px';
  }
}