Prevent navigation during file upload

I have a vue component for video upload, where I am warning a user when he tries to navigate away during the video upload that he will lose the file if he does so, like this:

ready() {
            window.onbeforeunload = () => {
                if (this.uploading && !this.uploadingComplete && !this.failed) {
                    this.confirm('Are you sure you want to navigate away? Your video won't be uploaded if you do so!');
                }
            }
        }

But how can I then make it stay on the same page, and prevent the navigation away before he confirms that he wants to navigate away?

This is the whole component:

<script>
    function initialState (){
        return {
            uid: null,
            uploading: false,
            uploadingComplete: false,
            failed: false,
            title: null,
            link: null,
            description: null,
            visibility: 'private',
            saveStatus: null,
            fileProgress: 0
        }
    }
    export default {
        data: function (){
            return initialState();
        },
        methods: {
            fileInputChange() {
                this.uploading = true;
                this.failed = false;

                this.file = document.getElementById('video').files[0];

                this.store().then(() => {
                    var form = new FormData();

                    form.append('video', this.file);
                    form.append('uid', this.uid);

                    this.$http.post('/upload', form, {
                        progress: (e) => {
                            if (e.lengthComputable) {
                                this.updateProgress(e)
                            }
                        }
                    }).then(() => {
                        this.uploadingComplete = true
                    }, () => {
                        this.failed = true
                    });
                }, () => {
                    this.failed = true
                })
            },
            store() {
                return this.$http.post('/videos', {
                    title: this.title,
                    description: this.description,
                    visibility: this.visibility,
                    extension: this.file.name.split('.').pop()
                }).then((response) => {
                    this.uid = response.json().data.uid;
                });
            },
            update() {
                this.saveStatus = 'Saving changes.';

                return this.$http.put('/videos/' + this.uid, {
                    link: this.link,
                    title: this.title,
                    description: this.description,
                    visibility: this.visibility
                }).then((response) => {
                    this.saveStatus = 'Changes saved.';

                    setTimeout(() => {
                        this.saveStatus = null
                    }, 3000)
                }, () => {
                    this.saveStatus = 'Failed to save changes.';
                });
            },
            updateProgress(e) {
                e.percent = (e.loaded / e.total) * 100;
                this.fileProgress = e.percent;
            },
            confirm(message) {
              swal({
                title: message,
                text: null,
                type: "warning",
                showCancelButton: true,
                cancelButtonText: "Cancel",
                cancelButtonColor: '#FFF',
                confirmButtonColor: "#2E112D",
                confirmButtonText: "Yes, delete"
              }).then(function(){
                  this.$data = initialState();
              }.bind(this), function(dismiss) {
                // dismiss can be 'overlay', 'cancel', 'close', 'esc', 'timer'
                if (dismiss === 'cancel') { // you might also handle 'close' or 'timer' if you used those
                // ignore
                } else {
                  throw dismiss;
                }
              })
            }
        },
        ready() {
            window.onbeforeunload = () => {
                if (this.uploading && !this.uploadingComplete && !this.failed) {
                    this.confirm('Are you sure you want to navigate away? Your video won't be uploaded if you do so!');
                }
            }
        }
    }
</script>