code: function checkDuration(val, name) {
    if (typeof val !== "number") {
      warn(
        `<transition> explicit ${name} duration is not a valid number - ` +
          `got ${JSON.stringify(val)}.`
      );
    } else if (isNaN(val)) {
      warn(
        `<transition> explicit ${name} duration is NaN - ` +
          "the duration expression might be incorrect."
      );
    }
  }
docs: only used in dev mode