Flow allow concatenation with object.entries and string

Object.entries is typed as:

static entries(object: $NotNullOrVoid): Array<[string, mixed]>

And when you try to use a value of a mixed type you must first figure out what the actual type is or you’ll end up with an error, so you can use Flow's type refinement :

Object.entries(my).forEach(entry => {
  if (typeof entry[1] === 'number') {
    mystring = entry[1] + mystring;
  }
});

You could also just cast entry[1]:

Object.entries(my).forEach(entry => {
  mystring = Number(entry[1]) + mystring;
});

or add type to entry like this entry: [string, any] but note that using any is completely unsafe, and should be avoided whenever possible.

DEMO