<input type="text" id="input">
<button onclick="copyText(document.getElementById('input').value)">复制文本</button>

<script>
  //1.通常写法
  function copyText(text) {
    if (navigator.clipboard) {
      navigator.clipboard.writeText(text);
    } else {
      var input = document.createElement('input');
      input.setAttribute('value', text);
      document.body.appendChild(input);
      input.select();
      document.execCommand('copy');
      document.body.removeChild(input);
    }
  }
  
  //2.惰性函数写法 一旦确定下来、就固定下来
  function copyText(text) {
    if (navigator.clipboard) {
      copyText = (text) => {
        navigator.clipboard.writeText(text);
      }
    } else {
      function copyText(text) {
        var input = document.createElement('input');
        input.setAttribute('value', text);
        document.body.appendChild(input);
        input.select();
        document.execCommand('copy');
        document.body.removeChild(input);
      }
    }
    copyText(text);
  }
  
  //3.函数进阶写法
  function createCopyText() {
    if (navigator.clipboard) {
      return (text) => {
        navigator.clipboard.writeText(text);
      }
    } else {
      return (text) => {
        var input = document.createElement('input');
        input.setAttribute('value', text);
        document.body.appendChild(input);
        input.select();
        document.execCommand('copy');
        document.body.removeChild(input);
      }
    }
  };
  const copyText = createCopyText();
  //4.永远不变的立即执行写法
  const copyText = (function () {
    if (navigator.clipboard) {
      return (text) => {
        navigator.clipboard.writeText(text);
      }
    } else {
      return (text) => {
        var input = document.createElement('input');
        input.setAttribute('value', text);
        document.body.appendChild(input);
        input.select();
        document.execCommand('copy');
        document.body.removeChild(input);
      }
    }
  })();

</script>