This is a simple code to add HTML to either side of links in Squarespace

addLinkHTML.js

/* 

Add Icons to HTML Links JavaScript 

Demo 

addHeaderHTML(getLink('/contact'),'before','<span>Cool</span>')
addHeaderHTML(getLink('/contact'),'before','<span>Cool</span>')

*/ 


var links = document.querySelectorAll('header a');

var getLink = function(link) {
    return document.querySelector(`[href="${link}"]`)
}

function addHeaderHTML(linkSelector, position, html) {
    if(position === 'before') {
        linkSelector.insertAdjacentHTML('afterbegin',html)
    } else if (position === 'after') {
        linkSelector.insertAdjacentHTML('beforeend',html)
    }

}