Even though the answer from @dzuc is already very good, you could use array destructuring (available in modern browsers or with Babel) to make it even a bit more elegant:
// original version from @dzuc
const data = Array.from(formData.entries())
.reduce((memo, pair) => ({
...memo,
[pair[0]: pair[1],
}), {})
// with array destructuring
const data = Array.from(formData.entries())
.reduce((memo,[key, value]) => ({
...memo,
[key]: value,
}), {})