_prepareValue(value) {
    if (typeof value === "number") {
      return value + "px";
    } else if (typeof value === "string") {
      if (value.indexOf("%") !== -1 || value.indexOf("px") !== -1) {
        return value;
      } else if (value.indexOf("%") === -1) {
        return value + "px";
      }
    }
    throw new Error(
      "Could not use the value supplied for width or height:" + value
    );
  }
@param {number|string} value
@returns {string}
@private