Validate local images based on their dimensions

Use the onFileSelected callback to enforce image dimension constraints for local files

Configure the picker options

Users can accept or reject selected files by using a special feature of the onFileSelected callback option to the picker. If the function throws an error then the file is rejected and a notification of the error message is displayed. But you can also return a Promise, and if the Promise rejects then the file is also rejected with a notification.

This support for Promises makes asynchronous validation possible, and this means we can check image dimensions for local files.

In order to make this happen we need to make sure the option exposeOriginalFile is true. This will ensure we can work with the actual File object that represents the user’s file on disk.

function checkImgSize (file) {
  return new Promise(function (resolve, reject) {
    const maxWidth = 1300;
    const maxHeight = 1300;
    const blob = file.originalFile;

    // Get an object URL for the local file
    const url = URL.createObjectURL(blob);

    // Create an image and load the object URL
    const img = new Image();
    img.src = url;

    // Don't forget to clean up after yourself
    URL.revokeObjectURL(url);

    img.onload = function () {

      if (this.naturalWidth > maxWidth) {
        reject('File is too wide');
      }

      if (this.naturalHeight > maxHeight) {
        reject('File is too tall');
      }

      // If we made it here then the file was approved
      resolve();
    }
  });
}

const picker = client.picker({ 
  exposeOriginalFile: true,
  onFileSelected: checkImgSize,
});

Using this simple function you make sure that local images are the right size before they get selected. In combination with maxSize and the image resizing options (imageMin, imageMax, imageDim), the picker provides a helpful set of features to ensure that your image requirements are met.

As a side note, loading the entire image into memory may not be optimal for your use case. It is also possible to read just a slice of the file to retrieve its EXIF data, if present, and use that to determine image dimensions. This can be accomplished inside of a web worker to avoid performance issues, and we will explore this in another tutorial.