Access this.data object inside nested method functions

Question

I'm new to Vue, I tried some similar potential solutions I found but none worked:

I need to update this.data_object from within a method, which contains nested functions. I can't figure out the inheritance.

I've tried this which doesn't exist, a copy of vm. from my main.js file, vm.$root and a few other combos.

Here is example code:

//this method is called on a file upload dropzone with :on-change="handlePreview"
handlePreview(file,files) {//file
   
    var reader = new FileReader();
    reader.onload = function (reader) {
        
        papaparse.parse(reader.target.result, {
            step: function(row) {
                console.log("Row:", row.data);
                //this is where i need to update data for this template. i defined this as data.preview_table{items:[]}
                vm.this.preview_table.items.push(row.data)
            }
        });

    }

    reader.readAsText(this.files[0].raw, "UTF-8");
   
}

Thanks for the help here is updated code where inheritance works:

handlePreview(file,files) {//file
   
    var reader = new FileReader();
    reader.onload = (reader) => {
        
        papaparse.parse(reader.target.result, {
            worker: true,
            //header:true,
            delimiter: ",",
            download: false,
            preview: 6,
            step: (row) => {
                console.log("Row:", row.data);
                console.log(this.is_preview_loaded)
                this.preview_table.items.push(row.data)
            },
            complete: (event) => {
                console.log(event)
            },
            error: (error,file) => {
                console.log(error)
                return swal.fire({
                    title: `Error parsing this file`,
                    text: 'Please double check this is a correct CSV file',
                    buttonsStyling: false,
                    icon: 'warning'
                })
            }
        });

    }

    reader.readAsText(this.files[0].raw, "UTF-8");
},