data（）中的错误：“ TypeError：无法读取未定义的属性'length'”（Vue.js）

Question

i am a beginner in Vue.js so maybe i made a stupid mistake but i honestly don't understand why do i got such an error... can you help me please? here is a code: Javascript:

const bootstrap = require("./assets/bootstrap.png");
const bulma = require("./assets/bulma.png");
const css3 = require("./assets/css3.png");
const html5 = require("./assets/html5.png");
const illustrator = require("./assets/illustrator.png");
const js = require("./assets/js.png");
const photoshop = require("./assets/photoshop.png");
const vue = require("./assets/vue.png");
const webpack = require("./assets/webpack.png");

export default {
  name: "app",
  data() {
    return {
      images: [
        bulma,
        bootstrap,
        css3,
        html5,
        illustrator,
        js,
        photoshop,
        vue,
        webpack
      ],
      idx: Math.floor(Math.random() * this.images.length),
      randomImage: this.images[this.idx]
    };
  }
};

and HTML:

  <div id="app">
    <div id="myContainer">
      <div id="nav">
        <router-link to="/">Home</router-link> |
        <router-link to="/about">About</router-link>
      </div>
      <router-view />
      <button v-on:click="animate">Test</button>
      <img v-for="image in images" :src="image" />
    </div>
  </div>

Error in data(): "TypeError: Cannot read property 'length' of undefined" (Vue.js)!!! problem is connected with Math.floor(Math.random() * this.images.length) as i understand. In future i want to use randomPicture to generate random pictures.

Answer 1

When you create your component with this :

export default {
  name: "app",
  data() {
    return {
      images: [
        bulma,
        bootstrap,
        css3,
        html5,
        illustrator,
        js,
        photoshop,
        vue,
        webpack
      ],
      idx: Math.floor(Math.random() * this.images.length),
      randomImage: this.images[this.idx]
    };
  }
};

this.images (or this.idx ) is not yet defined. You should set a value (like null ) to randomImage , then set the actual value on the created hook :

export default {
  name: "app",
  data() {
    return {
      images: [
        bulma,
        bootstrap,
        css3,
        html5,
        illustrator,
        js,
        photoshop,
        vue,
        webpack
      ],
      idx: null,
      randomImage: null
    };
  },
  created() {
    this.idx = Math.floor(Math.random() * this.images.length)
    this.randomImage = this.images[this.idx]
  }
};

Answer 2

You can use a computed property instead to data variables like this:

export default {
  name: "app",
  data() {
    return {
      images: [
        bulma,
        bootstrap,
        css3,
        html5,
        illustrator,
        js,
        photoshop,
        vue,
        webpack
      ],
    };
  },
  computed: {
    idx() {
      return Math.floor(Math.random() * this.images.length);
    },
    randomImage() {
      return this.images[this.idx];
    },
  },
};

Also you can only use the component data variables once the component is created or mounted.

Answer 3

装入组件时，您的数据为空（因为您是异步获取的），因此您需要一个额外的保护措施。

Answer 4

Like other answers indicate, this.images is not yet defined when you use it. So try this:

const bootstrap = require("./assets/bootstrap.png");
const bulma = require("./assets/bulma.png");
const css3 = require("./assets/css3.png");
const html5 = require("./assets/html5.png");
const illustrator = require("./assets/illustrator.png");
const js = require("./assets/js.png");
const photoshop = require("./assets/photoshop.png");
const vue = require("./assets/vue.png");
const webpack = require("./assets/webpack.png");

export default {
  name: "app",
  data() {
    const images = [
        bulma,
        bootstrap,
        css3,
        html5,
        illustrator,
        js,
        photoshop,
        vue,
        webpack
      ]
    const idx = Math.floor(Math.random() * images.length)
    return {
      images,
      idx,
      randomImage: images[idx]
    };
  }
};