html

<button type="button"  id="qckbh" >清除空白换行</button>

<textarea cols="20" rows="10"  id="kw"></textarea>

空白换行

function qckbh() {   let input = document.getElementById("kw");
        
        let text = input.value.replace(/\n{2,}/g, '\n');
      
        input.value = text; 
};

所有换行

  function qcsyhh() {
    let input = document.getElementById("kw");
    let text = input.value;
    
    text = text.replace(/(\r\n|\n|\r)/gm, "");
  
    input.value = text;
  };

空格和换行
-----

    function qchhkg() {
      let input = document.getElementById("kw");
      
      let text = input.value.replace(/\s+/g, "");
    
      input.value = text; 
    };

所有空格

function qckg() {if(document.getElementById("kw").value.indexOf(" ")>=0){

    var str = document.getElementById("kw").value;
    
    var reg1 = new RegExp(" ","g"); 
    var a1 = str.replace(reg1,"");
    
    document.getElementById('kw').value=a1;
    
    }
    else {
    alert("没有空格");
    }};