如何使用 Jest 测试 asnyc 代码(或使用 jsdom 测试“image.onload") [英] How to test asnyc code with Jest ( or test "image.onload" with jsdom )

问题描述

:我已经用承诺方式更改了我的代码.

: I have change my code with promise way .

我正在写与 this 由 facebook 创建的 starter 的反应，我是测试新手.

I am writing react with this starter created by facebook, and I'm a newbie about testing.

现在我有一个关于图像的组件，它有一个检查图像大小的功能:

Now I have a component about image, it has a function to check Image size:

import React, { Component } from 'react';


class ImagePart extends Component {
    .....
    //  check size.
    checkSize(src, width, height){
        this.loadImg(src)
        .then((obj) => {
            return (obj.width >= width && obj.height >= height)
            ? true : false;
        })
        .catch((msg)=> {
            dosomething
        });
    }
    // load image and return a promise.
    loadImg(src){
        return new Promise((resolve, reject) => {
            let imageObj = new Image();
            imageObj.onload = (evt) => {
                resolve(evt.target);
            }
            imageObj.error = (err) =>{
                reject(err);
            }
            imageObj.src = src; 
        })
    }
    .....
}

和测试片段:

import React from 'react';
import ReactDOM from 'react-dom';
import ImagePart from './ImagePart';



it('checking image size without error', () => {
    const image = new ImagePart();
    const img300_300 = 'https://someImage.png';
    expect(image.loadImg(img300_300).width).resolves.toBe(300);
    // ??? test checkSize
});

运行测试后，我收到此错误:

After Running the test, I got this error :

TypeError: 无法读取未定义的属性 'toBe'

TypeError: Cannot read property 'toBe' of undefined

问题是:

1. 如何以正确的方式测试 `loadImg?
2. 测试 checkSize 的一般模式是什么?

谢谢.

推荐答案

checkSize 的当前实现是异步的，总是返回 undefined.

The current implementation of checkSize is asynchronous and always returns undefined.

您应该使用 回调 或返回 Promise.

You should either use a callback or return a Promise.

function checkSizeWithCallback(src, width, height, callback) {
  const image = new Image();
  image.onload = evt => {
    const result = evt.target.width >= width && evt.target.height >= height;
    callback(null, result);
  };
  image.onerror = // TODO: handle onerror
  image.src = src; 
}


it('...', done => {
  checkSizeWithCallback(/* args */, (err, result) => {
    expect(result).toEqual(true);
    done(err);
  });
});

<小时>

function checkSizeWithPromise(src, width, height) {
  return new Promise((resolve, reject) => {
    const image = new Image();
    image.onload = evt => {
      const result = evt.target.width >= width && evt.target.height >= height;
      resolve(result);
    };
    image.onerror = // TODO: handle onerror
    imageObj.src = src; 
  });
}


it('...', () => {
  return checkSizeWithPromise(/* args */)
    .then(result => {
      expect(result).toEqual(true);
  });
});

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