Peer review

// This is an improved version of https://stackoverflow.com/a/5835533/1175714

function slideLoop(currMarg, selector){
    let contStyle = document.querySelector(selector).style;

    return function(){
        currMarg = (currMarg == 1800) ? (0) : (currMarg + 600);
        contStyle.marginLeft = "-%dpx".replace("%d", curMarg);
    }
}

function slide() {
    return setInterval(slideLoop(0, "#container"), 3000);
}


// Notes:
// Use document.querySelector(css_selector) instead of document.getElementById(id)
// Always track setInterval(fn, interval)'s return value so it can later be closed with clearInterval(interval_id)
// Put the interval's callback into its own formal function for readability and maintainability
// If state is kept with the callback, the callback should be returned from another function (slideLoop) to form a closure.
// Use proper whitespace. Logical blocks that do different things should be separated by one empty line
// Avoid "string" + value + "string" + ... Make a format function or use String.prototype.replace(string/regex, new_string)
// Always surround the arguments of your ternary expressions with parens. It aids readability and prevents bugs.