JavaScript

(function () {
  const setSize = 360; //　希望のviewport下限を設定
  const viewport = document.querySelector("meta[name='viewport']");
  function adjustViewport() {
    /*
    三項演算子(条件演算子)
    condition ? exprIfTrue(trueに変換できる値) : exprIfFalse(falseに変換できる値)
    */
    const value = window.outerWidth > setSize ? "width=device-width,initial-scale=1" : "width=" + setSize;
    if (viewport.getAttribute("content") !== value) {
      viewport.setAttribute("content", value);
    }
  }
  addEventListener("resize", adjustViewport);
  adjustViewport();
})();