this.propertyis undefined： 无法读取 undefined 的属性'classList'

Question

我正在制作一些东西来可视化照片
目标是在"列表"中选择您想要的图片，使其出现在主HTML元素上。但是为了帮助你找到你在列表中的位置，有一个类在你选择的元素上加上了边框。

问题：
用事件this.block.onclick = function ()执行的函数开始得很好，.selected从最初选择的元素中删除，但当this.block.classList.add('selected');出现时，我得到了这个错误：

media_visu.js:26未捕获类型错误：无法读取未定义的属性"classList">

我试着把function放在外面，试着className，setAttribute，但什么都没有改变：我的this.block似乎是undefined。

mediavisu.js:

var mediaVisu = function () {
    'use strict';
    window.console.log('mediaVisu loaded');


    var i,
    visu = document.querySelector("#img"),
    Album = [];
    function Photo(nb, folder) {
        this.block = document.querySelector("#list_img_" + nb);
        this.place = 'url(../src/' + folder + '/' + nb + '.jpg)';
        this.block.onclick = function () {
            for (i = 0; i < Album.length; i += 1) {
                window.console.log(Album[i].block);
                if (Album[i].block.classList.contains('selected')) {
                    Album[i].block.classList.remove('selected');
                }
            }
            visu.style.background = this.place;
            window.console.log(visu.style.background);
            window.console.log(this.place);
            this.block.classList.add('selected');
        };
        Album[Album.length] = this;
    }
    var test_a = new Photo(1, "test"),
    test_b = new Photo(2, "test"),
    test_c = new Photo(3, "test"),
    test_d = new Photo(4, "test"),
    test_e = new Photo(5, "test");
    window.console.log(Album);
    for (i = 0; i < Album.length; i += 1) {
        window.console.log(Album[i]);
    }
};

Answer 1

在onclick函数中，this将是点击的元素

所以你可以简单地使用

this.classList.add('selected');

你可能需要重新考虑使用this.place，因为this不会是你认为的this。。一个常见的解决方案如下

function Photo(nb, folder) {
    this.block = document.querySelector("#list_img_" + nb);
    this.place = 'url(../src/' + folder + '/' + nb + '.jpg)';
    var self = this;
    this.block.onclick = function () {
        for (i = 0; i < Album.length; i += 1) {
            window.console.log(Album[i].block);
            if (Album[i].block.classList.contains('selected')) {
                Album[i].block.classList.remove('selected');
            }
        }
        visu.style.background = self.place;
        window.console.log(visu.style.background);
        window.console.log(self.place);
        this.classList.add('selected');
    };
    Album[Album.length] = this;
}

或者，使用绑定

function Photo(nb, folder) {
    this.block = document.querySelector("#list_img_" + nb);
    this.place = 'url(../src/' + folder + '/' + nb + '.jpg)';
    this.block.onclick = function () {
        for (i = 0; i < Album.length; i += 1) {
            window.console.log(Album[i].block);
            if (Album[i].block.classList.contains('selected')) {
                Album[i].block.classList.remove('selected');
            }
        }
        visu.style.background = this.place;
        window.console.log(visu.style.background);
        window.console.log(this.place);
        this.block.classList.add('selected');
    }.bind(this);
    Album[Album.length] = this;
}

注意：现在返回到this.block.classList.add('selected')，因为this现在是之前所期望的this

Answer 2

您可以使用"this"(如前一个答案中所述(或使用事件目标：访问它

    this.block.onclick = function (e) {
        for (i = 0; i < Album.length; i += 1) {
            window.console.log(Album[i].block);
            if (Album[i].block.classList.contains('selected')) {
                Album[i].block.classList.remove('selected');
            }
        }
        visu.style.background = this.place;
        window.console.log(visu.style.background);
        window.console.log(this.place);
        e.target.classList.add('selected');
    };