send base64 string data to server but receive empty form value - google-app-engine

Question

I have a front-end written in Vue and a backend written in Golang. I'm using Google app engine to run my backend service, and use gcloud datastore and gcloud storage to store the data and image that were submitted through front-end form.
I've been trying to upload an image using POST method. I convert the image to a base64 string. Then I add the data string to formdata and POST to my backend service. I keep getting empty form value in Go program. Is there a reason that Go cannot read base64 string, or I miss something important about FormData? Any help helps, thank you.
My front-end code:
var myForm = document.getElementById('myForm')
var formData = new FormData(myForm)
var imgBase64 = getBase64(//image-url//)
imgBase64.then(function (res) {
formData.append('image', res)
}
axios.post(' //go-app-engine-service// ', formData)
.then(res => {
console.log(res)
})
.catch(error => {
console.log(error)
})
function getBase64(url) {
return axios
.get(url, {
responseType: 'arraybuffer'
})
.then(response => Buffer.from(response.data, 'binary').toString('base64'))}
My Go code:
imgString := r.FormValue("image")
fmt.Printf("imgstring: %s, %d, %T\n", imgString, len(imgString), imgString) //=> getting empty imgString

Answer 1

Ok, after some research I realize the "scope" issue.
function getBase64 returns a Promise and have to handle the value inside the scope, so I move the axios.post in the Promise and I finally see the base64 value in Go program. Problem solved.
modified front-end code:
var myForm = document.getElementById('myForm')
var formData = new FormData(myForm)
var imgBase64 = getBase64(//image-url//)
imgBase64.then(function (res) {
formData.append('image', res)
axios.post(' //go-app-engine-service// ', formData)
.then(res => {
console.log(res)
})
.catch(error => {
console.log(error)
})
}