[persi005]

Если кому интересно, то подсократил код:

(function() {

    var HIDDEN_CLASS = "hidden";
    var OVERFLOW_BLOCK = "overflow-block";
    var MODAL = document.body.querySelector(".modal");
    var MODAL_IMAGE = MODAL.querySelector("img");
    var MODAL_CAPTION = MODAL.querySelector("p");
    var MODAL_BUTTON_CLOSE = MODAL.querySelector(".modal-close");
    var MODAL_BUTTON_BACK = MODAL.querySelector(".modal-previous");
    var GALLERY_ARRAY = document.body.querySelectorAll(".photos img");
    var CURRENT_IMG_ID;

    var actionImgModal = {

        openImg(modalWindow, modalImg, modalCaption, imagesArray) {
            for (var i = 0; i < imagesArray.length; i++) {
                imagesArray[i].id = i;
                imagesArray[i].onclick = function () {
                    CURRENT_IMG_ID = this.id;
                    modalImg.src = this.src;
                    modalCaption.innerHTML = this.alt;
                    modalWindow.classList.remove(HIDDEN_CLASS);
                    document.body.classList.add(OVERFLOW_BLOCK);
                };
            } 
        },

        closeImg(modalWindow) {
            document.body.classList.remove(OVERFLOW_BLOCK);
            modalWindow.classList.add(HIDDEN_CLASS);
        },

        changeImg(modalImg, modalCaption, imagesArray) {

            if (CURRENT_IMG_ID == imagesArray.length) { CURRENT_IMG_ID = 0;
            } else if (CURRENT_IMG_ID == -1) { CURRENT_IMG_ID = imagesArray.length - 1
            }
            modalImg.src = imagesArray[CURRENT_IMG_ID].src;
            modalImg.alt = imagesArray[CURRENT_IMG_ID].alt;
            modalCaption.innerHTML = modalImg.alt;
        },

        nextImg(modalImg, modalCaption, imagesArray) {
            CURRENT_IMG_ID++;
            this.changeImg(modalImg, modalCaption, imagesArray);
        },

        previousImg(modalImg, modalCaption, imagesArray) {
            CURRENT_IMG_ID--;
            this.changeImg(modalImg, modalCaption, imagesArray);
        }

    };

    actionImgModal.openImg(MODAL, MODAL_IMAGE, MODAL_CAPTION, GALLERY_ARRAY);

    MODAL_BUTTON_CLOSE.onclick = function () {
        actionImgModal.closeImg(MODAL);
    };

    MODAL_IMAGE.onclick = function () {
        actionImgModal.nextImg(MODAL_IMAGE, MODAL_CAPTION, GALLERY_ARRAY);
    };

    MODAL_BUTTON_BACK.onclick = function () {
        actionImgModal.previousImg(MODAL_IMAGE, MODAL_CAPTION, GALLERY_ARRAY);
    };

    document.onkeydown = function (evt) {
        if (evt.key == "Escape") {
            actionImgModal.closeImg(MODAL);
        } else if (evt.key == "ArrowRight") {
            actionImgModal.nextImg(MODAL_IMAGE, MODAL_CAPTION, GALLERY_ARRAY);
        } else if (evt.key == "ArrowLeft") {
            actionImgModal.previousImg(MODAL_IMAGE, MODAL_CAPTION, GALLERY_ARRAY);
        }
    };

})();