`extend` function for javascript objects

extend.js

/**
  * @desc From https://gomakethings.com/merging-objects-with-vanilla-javascript/
  */
function extend() {
    // Variables
    var extended = {};
    var deep = false;
    var i = 0;
    var length = arguments.length;

    // Check if a deep merge
    if ( Object.prototype.toString.call( arguments[0] ) === '[object Boolean]' ) {
        deep = arguments[0];
        i++;
    }

    // Merge the object into the extended object
    var merge = function ( obj ) {
        for ( var prop in obj ) {
            if ( Object.prototype.hasOwnProperty.call( obj, prop ) ) {
                // If deep merge and property is an object, merge properties
                if ( deep && Object.prototype.toString.call(obj[prop]) === '[object Object]' ) {
                    extended[prop] = extend( true, extended[prop], obj[prop] );
                } else {
                    extended[prop] = obj[prop];
                }
            }
        }
    };

    // Loop through each object and conduct a merge
    for ( i=0; i < length; i++ ) {
        var obj = arguments[i];
        merge(obj);
    }

    return extended;
}