Leveraging the FileReader API within a Vue.js component function

I'm working on a Vue.js file picker that needs to display previews of selected files. To accomplish this, I am utilizing the FileReader API to read user-selected files as data URLs using the readAsDataURL method of the FileReader object.

However, I'm encountering an error that states "Uncaught TypeError: reader.onload is not a function" when I try to use reader.onload in my handleFileChanges function.

It seems like the issue may be that the reader object is not defined, leading to the FileReader undefined error mentioned earlier.

Here is the snippet of how I am attempting to achieve this:

handleFileChanges (e) {
    var reader = new window.FileReader() // Using window to avoid FileReader undefined error
                reader.onload(function (event) {
                  let imageDataURL = event.target.result
                  this.$store.dispatch('attachedFile', imageDataURL)
                })
                reader.readAsDataURL(e.target.files[i])
}

Can someone please point out what I may be missing in my implementation?

Answer №1

It's important to note that the error message is accurate in stating that onload is not a function. This is because onload is an event handler/property that is assigned to a newly created instance of FileReader. As a result, it does not accept any callbacks.

To resolve this issue, try using the following code snippet:

handleFileChanges (e) {
    var reader = new window.FileReader() // Use window to prevent "File READER is not defined" error
                reader.onload = function (event) {
                  // Send event.target.result as imageDataURL to the state UI postEditor using fileAttached
                  let imageDataURL = event.target.result
                  this.$store.dispatch('attachedFile', imageDataURL) // Alternatively, use previewFile
                }
                reader.readAsDataURL(e.target.files[i])
}

Additionally, ensure that the context of this in this.$store.dispatch is correctly bound.

Answer №2

Since the onload property is used as a callback, it is recommended to set it accordingly. Even though FileReader is a subclass of EventTarget, you can use events like onload or onabort by utilizing the addEventListener method. If you need to pass a callback for the onload event handler, it's best to consider using addEventListener.

// method 1
reader.onload = function (e) { ...
// method 2
reader.addEventListener("onload", function (e) { ...

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