Turned/reversed the unpacker for WHAK compression algorithm and made it into a HTML entities encoder - http://www.whak.ca/packer/JavaScript.htm

html.entity.safe.escape.js

var W,H,A,K;
    H ="& < > " 
 '"[$="split"](" ");
    A ="&<>\"\n'";W='<script>alert("www.WHAK.com");<\/script>';
    K =0;for(K in A)W=W[$](A[K]).join(H[K]);alert(W);

var W,H,A,K;
H='Some <textarea id="test">www.WHAK.com</textarea>, more Text&Text';
K=[
    [/&/g,"&amp;"],
    [/</g,"&lt;"],
    [/>/g,"&gt;"],
    [/"/g,"&#34;"]
];
for(W in K)H=H.replace(K[W][0],K[W][1]);
alert(H);

Here is is as a functional function...

function HTML( W,H,A,K ){
    H = '& < > " 
 ''.split(' ');
    A = '&<>"\x0A\''.split('');
    K =  0; for(K in A)W=W.split(A[K]).join(H[K]);return W
}alert(HTML('<script>alert("www.WHAK.com");<\/script>'));

Minified

function HTM(W,H,A,K){H="&amp; &lt; &gt; &#34; &#10; &#39;"[x='split'](" ");A="&<>\"\n'"[x]("");K=0;for(K in A)W=W[x](A[K]).join(H[K]);return W}