flow-example.js

// no types
function getFields(fields) {
   return fields.reduce((prev, curr) => (prev[curr] = curr, prev), {})
}

// no return type
function getFields(fields: Array<string>) {
   return fields.reduce((prev, curr) => (prev[curr] = curr, prev), {})
}

// generic string: string mapping
function getFields(fields: Array<string>): { [key: string]: string } {
   return fields.reduce((prev, curr) => (prev[curr] = curr, prev), {})
}

// TODO: figure out how to get a return type that understands the strings
// When `getFields(['a', 'b'])` then the return type should be `{ a: string, b: string }`
function getFields(fields: Array<string>) {
   return fields.reduce((prev, curr) => (prev[curr] = curr, prev), {})
}