inputString(): string {
    if (this.input instanceof HTMLInputElement) {
      return this.input.value.toLowerCase();
    }
    if (this.input.isContentEditable) {
      return this.input.textContent.toLowerCase();
    }
    throw new Error(errors.invalidInput);
  }
The current search input in lower case
@function inputString
@return {String} the input as a string
@throws {Error} If The current <code>input</code> is no <code>&lt;input&gt;</code> or <code>contenteditable</code>
@memberOf holmes