[JavaScript知识库]react 上传 image

极简写法，只需要两个元素就足够，一个 input, 一个 button。
input 需要设置 type = "file", 然后添加一个 onChange listener， 用户选择任何文件，selectedFile 将被设为新选择的文件，点击 “Upload to Server” 上传selectedFile到后端。

代码如下：

import React, { useState } from "react";
import "./App.css";

function App() {
  const [selectedFile, setSelectedFile] = useState(null);
  
  const selecteFileHandler = (event) => {
    setSelectedFile(event.target.files[0]);  // 选择单个文件
  };

  const uploadHandler = () => {
    console.log("The file to be sent is:", selectedFile.name);
	// 将图像发送到后端，使用 fetch 或者 axios 都行
    const formData = new FormData();
    formData.append('image', selectedFile);
    axios.post("some backend end api point.....", formData)
    .then(res => {
    	console.log(res)
    })
  };

  return (
    <div className="App">
      <input type="file" onChange={selecteFileHandler} />
      <button onClick={uploadHandler}>Upload to Server</button>
    </div>
  );
}
export default App;

复杂一点的写法：主要是因为上面的 input 元素默认样式比较 ugly ，一般将其隐藏，新增一个 button，然后，每次点击这个 button，被隐藏的 input 执行其原来的功能。

代码：

import React, { useRef, useState } from "react";
import "./App.css";

function App() {
  const [selectedFile, setSelectedFile] = useState(null);
  const hiddenFileInputRef = useRef(null);

  const selecteFileHandler = (event) => {
    setSelectedFile(event.target.files[0]);
  };

  const uploadHandler = () => {
    console.log("The file to be sent is:", selectedFile.name);

    // send data to back end with fetch or axios!
    const formData = new FormData();
    formData.append("image", selectedFile, selectedFile.name);
    axios
      .post("some backend end point.....", formData, {
        // 获取上传进度:
        onUploadProgress: (progressEvent) => {
          console.log(
            "Upload Progress: " +
              Math.round((ProgressEvent.loaded / progressEvent.total) * 100) +
              "%"
          );
        },
      })
      .then((res) => {
        console.log(res);
      });
  };

  const pickImageHandler = () => {
    hiddenFileInputRef.current.click();
  };

  return (
    <div className="App">
      <input
        style={{ display: "none" }}
        type="file"
        onChange={selecteFileHandler}
        ref={hiddenFileInputRef}
      />
      <button onClick={pickImageHandler}>Pick a Image</button>
      <button onClick={uploadHandler}>Upload to Server</button>
    </div>
  );
}

export default App;

参考网页：

1. 油鹳，
2. https://medium.com/web-dev-survey-from-kyoto/how-to-customize-the-file-upload-button-in-react-b3866a5973d8
3. https://www.pluralsight.com/guides/how-to-use-a-simple-form-submit-with-files-in-react
4. https://www.geeksforgeeks.org/file-uploading-in-react-js/