How to convert HTML escape characters with JS

function removeHtmlTab(tab) {
 return tab.replace(/<[^<>]+?>/g,&#39;&#39;);//删除所有HTML标签
}

function html2Escape(sHtml) {
 return sHtml.replace(/[<>&"]/g,function(c){return {&#39;<&#39;:&#39;&lt;&#39;,&#39;>&#39;:&#39;&gt;&#39;,&#39;&&#39;:&#39;&amp;&#39;,&#39;"&#39;:&#39;&quot;&#39;}[c];});
}

function escape2Html(str) {
 var arrEntities={&#39;lt&#39;:&#39;<&#39;,&#39;gt&#39;:&#39;>&#39;,&#39;nbsp&#39;:&#39; &#39;,&#39;amp&#39;:&#39;&&#39;,&#39;quot&#39;:&#39;"&#39;};
 return str.replace(/&(lt|gt|nbsp|amp|quot);/ig,function(all,t){return arrEntities[t];});
}

function nbsp2Space(str) {
 var arrEntities = {'nbsp' : ' '};
 return str.replace(/&(nbsp);/ig, function(all, t){return arrEntities[t]})
}

function return2Br(str) {
 return str.replace(/\r?\n/g,"<br />");
}

function trimBr(str) {
 str=str.replace(/((\s| )*\r?\n){3,}/g,"\r\n\r\n");//限制最多2次换行
 str=str.replace(/^((\s| )*\r?\n)+/g,'');//清除开头换行
 str=str.replace(/((\s| )*\r?\n)+$/g,'');//清除结尾换行
 return str;
}

function mergeSpace(str) {
 str=str.replace(/(\s| )+/g,' ');
 return str;
}