Fastest method to escape HTML tags as HTML entities?

Here's one way you can do this:

var escape = document.createElement('textarea');
function escapeHTML(html) {
    escape.textContent = html;
    return escape.innerHTML;
}

function unescapeHTML(html) {
    escape.innerHTML = html;
    return escape.textContent;
}

Here's a demo.

You could try passing a callback function to perform the replacement:

var tagsToReplace = {
    '&': '&',
    '<': '&lt;',
    '>': '&gt;'
};

function replaceTag(tag) {
    return tagsToReplace[tag] || tag;
}

function safe_tags_replace(str) {
    return str.replace(/[&<>]/g, replaceTag);
}

Here is a performance test: http://jsperf.com/encode-html-entities to compare with calling the replace function repeatedly, and using the DOM method proposed by Dmitrij.

Your way seems to be faster...

Why do you need it, though?