Merge Two Sorted Lists in JavaScript
Looks good!

I would say maybe the following styles would be just a bit more readable, easier to follow.

We can also alter l1 and l2 with more descriptive variable names.


Line Counting Fallacy:

Sometimes, Line/character countings are helpful for command line languages/scripts (awk, grep, sed, regex, etc.) or maybe Code Golfing, is not a JavaScript practice though.

Iterative using a Sentinel Node
const mergeTwoLists = function(l1, l2) {
    const sentinel = {
        val: -1,
        next: null
    }

    let head = sentinel
    while (l1 &amp;&amp; l2) {
        if (l1.val &gt; l2.val) {
            head.next = l2
            l2 = l2.next
        } else {
            head.next = l1
            l1 = l1.next
        }
        
        head = head.next
    }

    head.next = l1 || l2

    return sentinel.next
}



Recursive
const mergeTwoLists = function(l1, l2) {
    if (l1 === null) {
        return l2
    }

    if (l2 === null) {
        return l1
    }

    if (l1.val &lt; l2.val) {
        l1.next = mergeTwoLists(l1.next, l2)
        return l1

    } else {
        l2.next = mergeTwoLists(l1, l2.next)
        return l2
    }
}

Or even better that those:
const mergeTwoLists = function(l1, l2) {
    if (l1 === null) {
        return l2;
    }

    if (l2 === null) {
        return l1;
    }

    if (l1.val &lt; l2.val) {
        l1.next = mergeTwoLists(l1.next, l2)
        return l1;

    } else {
        l2.next = mergeTwoLists(l1, l2.next)
        return l2;
    }
}




const mergeTwoLists = function(l1, l2) {
    const sentinel = {
        val: -1,
        next: null
    };

    let head = sentinel;
    while (l1 &amp;&amp; l2) {
        if (l1.val &gt; l2.val) {
            head.next = l2;
            l2 = l2.next;
        } else {
            head.next = l1;
            l1 = l1.next;
        }
        
        head = head.next;
    }

    head.next = l1 || l2;

    return sentinel.next;
}