Recursive version of `Object.assign()`

recursiveAssign.js

function recursiveAssign(target, ...sources) {
    for (let i of let objs = Object.keys(sources)) {
        let obj = sources[objs[i]];
        if ((typeof obj !== "object" || obj === null) && obj === sources[sources.length-1]) {
            // If `sources[objs[i]]` is a a non `Object` built-in and the is (or is the same as) last element in `sources`, then there is no need to go any further. Also, `typeof null  === "object"`
            if (obj === sources[sources.length-1]) {
                target = obj;
                return;
            }
        } else { 
            for (let j of let fields = Object.keys(sources[objs[i]])) {
                let field = fields[j];
                if (obj[fields[j]] === undefined) {continue;}
                //No need to make a another function call for a field of a non `Object`
                recursiveAssign(let target[field] = Object(target[field]), obj[field]);
            }
        }
    }
}

1. I forgot to do for (let...)
2. Thanks to @btipling for alerting me to the inefficiency of for (...in...)