Added longwinded comment to explain the reasoning behind #465

Author: erikras

src/Field.test.js

@@ -290,7 +290,9 @@ describe('Field', () => {
                   {...input}
                   data-testid="name"
                   onBlur={e => {
-                    input.onChange(e.target.value && e.target.value.toUpperCase())
+                    input.onChange(
+                      e.target.value && e.target.value.toUpperCase()
+                    )
                     input.onBlur(e)
                   }}
                 />

src/useField.js

@@ -119,7 +119,16 @@ function useField<FormValues: FormValuesShape>(
       (event: ?SyntheticFocusEvent<*>) => {
         state.blur()
         if (formatOnBlur) {
+          /**
+           * Here we must fetch the value directly from Final Form because we cannot
+           * trust that our `state` closure has the most recent value. This is a problem
+           * if-and-only-if the library consumer has called `onChange()` immediately
+           * before calling `onBlur()`, but before the field has had a chance to receive
+           * the value update from Final Form.
+           */
           const fieldState = form.getFieldState(state.name)
+          // this ternary is primarily to appease the Flow gods
+          // istanbul ignore next
           state.change(
             format(fieldState ? fieldState.value : state.value, state.name)
           )