src/utils/src-loader.js
function checkIsImage (src, onResult) {
  var request;

  if (src.tagName) {
    onResult(src.tagName === 'IMG');
    return;
  }
  request = new XMLHttpRequest();

  // Try to send HEAD request to check if image first.
  request.open('HEAD', src);
  request.addEventListener('load', function (event) {
    var contentType;
    if (request.status >= 200 && request.status < 300) {
      contentType = request.getResponseHeader('Content-Type');
      if (contentType == null) {
        checkIsImageFallback(src, onResult);
      } else {
        if (contentType.startsWith('image')) {
          onResult(true);
        } else {
          onResult(false);
        }
      }
    } else {
      checkIsImageFallback(src, onResult);
    }
    request.abort();
  });
  request.send();
}
function checkIsImage (src, onResult) {
  var request;

  if (src.tagName) {
    onResult(src.tagName === 'IMG');
    return;
  }
  request = new XMLHttpRequest();

  // Try to send HEAD request to check if image first.
  request.open('HEAD', src);
  request.addEventListener('load', function (event) {
    var contentType;
    if (request.status >= 200 && request.status < 300) {
      contentType = request.getResponseHeader('Content-Type');
      if (contentType == null) {
        checkIsImageFallback(src, onResult);
      } else {
        if (contentType.startsWith('image')) {
          onResult(true);
        } else {
          onResult(false);
        }
      }
    } else {
      checkIsImageFallback(src, onResult);
    }
    request.abort();
  });
  request.send();
}
function checkIsImage ( src , onResult ) { var request ; if ( src . tagName ) { onResult ( src . tagName === 'IMG' ) ; return ; } request = new XMLHttpRequest ( ) ; // Try to send HEAD request to check if image first. request . open ( 'HEAD' , src ) ; request . addEventListener ( 'load' , function ( event ) { var contentType ; if ( request . status >= 200 && request . status < 300 ) { contentType = request . getResponseHeader ( 'Content-Type' ) ; if ( contentType == null ) { checkIsImageFallback ( src , onResult ) ; } else { if ( contentType . startsWith ( 'image' ) ) { onResult ( true ) ; } else { onResult ( false ) ; } } } else { checkIsImageFallback ( src , onResult ) ; } request . abort ( ) ; } ) ; request . send ( ) ; }
Call back whether `src` is an image.

@param {string|Element} src - URL or element that will be tested.
@param {function} onResult - Callback with whether `src` is an image.
Call back whether src is an image .