JavaScript上传前获取文件大小，图像宽度和高度

Question

在使用jQuery或JavaScript上传到我的网站之前，如何获取文件大小，图像高度和宽度？

Answer 1

使用 HTML5和File API

图片来源将是代表Blob对象的URL
<img src="blob:null/026cceb9-edr4-4281-babb-b56cbf759a3d">

const EL_browse  = document.getElementById('browse');

const EL_preview = document.getElementById('preview');



const readImage  = file => {

  if ( !(/^image\/(png|jpe?g|gif)$/).test(file.type) )

    return EL_preview.insertAdjacentHTML('beforeend', `Unsupported format ${file.type}: ${file.name}<br>`);



  const img = new Image();

  img.addEventListener('load', () => {

    EL_preview.appendChild(img);

    EL_preview.insertAdjacentHTML('beforeend', `<div>${file.name} ${img.width}×${img.height} ${file.type} ${Math.round(file.size/1024)}KB<div>`);

    window.URL.revokeObjectURL(img.src); // Free some memory

  });

  img.src = window.URL.createObjectURL(file);

}



EL_browse.addEventListener('change', ev => {

  EL_preview.innerHTML = ''; // Remove old images and data

  const files = ev.target.files;

  if (!files || !files[0]) return alert('File upload not supported');

  [...files].forEach( readImage );

});


#preview img { max-height: 100px; }


<input id="browse" type="file" multiple>

<div id="preview"></div>

使用FileReader API的

如果您需要图像源作为长Base64编码的数据字符串
<img src="data:image/png;base64,iVBORw0KGg... ...lF/++TkSuQmCC=">

const EL_browse  = document.getElementById('browse');

const EL_preview = document.getElementById('preview');



const readImage = file => {

  if ( !(/^image\/(png|jpe?g|gif)$/).test(file.type) )

    return EL_preview.insertAdjacentHTML('beforeend', `<div>Unsupported format ${file.type}: ${file.name}</div>`);



  const reader = new FileReader();

  reader.addEventListener('load', () => {

    const img  = new Image();

    img.addEventListener('load', () => {

      EL_preview.appendChild(img);

      EL_preview.insertAdjacentHTML('beforeend', `<div>${file.name} ${img.width}×${img.height} ${file.type} ${Math.round(file.size/1024)}KB</div>`);

    });

    img.src = reader.result;

  });

  reader.readAsDataURL(file);

};



EL_browse.addEventListener('change', ev => {

  EL_preview.innerHTML = ''; // Clear Preview

  const files = ev.target.files;

  if (!files || !files[0]) return alert('File upload not supported');

  [...files].forEach( readImage );

});


#preview img { max-height: 100px; }


<input id="browse" type="file"  multiple>

<div id="preview"></div>