请注意，您需要反引号引用嵌入换行符的字符串

I have this string:

var str = '<p>paragraph<a>link</a></p>
           <div class="myclass">
               <div>something</div>
               <div style="mystyle">something</div>
               <b><a href="#">link</a></b>
               <b><a href="#" name="a name">link</a></b>
               <b style="color:red">bold</b>
               <img src="../path" alt="something" />
               <img src="../path" alt="something" class="myclass" />
           </div>';

I want to remove all attributes except href, src, alt. So this is expected result:

/* 
<p>paragraph<a>link</a></p>
<div>
    <div>something</div>
    <div>something</div>
    <b><a href="#">link</a></b>
    <b><a href="#">link</a></b>
    <b>bold</b>
    <img src="../path" alt="something">
    <img src="../path" alt="something">
</div>

How can I do that?

I can just select them which isn't useful:

/<[a-z]+ .*?(href|src|alt)="[^"]+"/g

var str = `<p>paragraph<a>link</a></p>
           <div class="myclass">
               <div>something</div>
               <div style="mystyle">something</div>
               <b><a href="#">link</a></b>
               <b><a href="#" name="a name">link</a></b>
               <b style="color:red">bold</b>
               <img src="../path" alt="something" />
               <img src="../path" alt="something" class="myclass" />
           </div>`;
var div = document.createElement("div");
div.innerHTML=str;
div.querySelectorAll("*").forEach(function(el){
  for (var i = 0, atts = el.attributes, n = atts.length; i < n; i++){
    var att = atts[i].nodeName;
    if (["src","alt","href"].indexOf(att) ==-1) el.removeAttribute(att); 
  }
}); 
// console.log(div); alert shows it more clearly
alert(div.innerHTML);

PS: Please note that you need backticks to quote a string with embedded newlines