input's event.target is null within this.setState [React.js]

The setState function is executed in asynchronous context.

By the time the state is updated the e.target reference might or might not be gone.

const file = e.target.files[0]; can be used to "remember" the value as in your example.

What is the reason for calling setState with callback ? this.setState({ file: e.target.files[0] }) should do the job.

In your code you are referring to a synthetic event object which no longer holds information about the original DOM event. React reuses the event objects for performance reasons.

Alternatively you can use:

let file = e.target.files[0]; const files = e.target.files this.setState(() => ({ file: files[0] })); //doesnt work

React uses event pooling, you can read more about it in the docs here https://reactjs.org/docs/events.html

setState is an asynchronous function

this.setState(() => ({ file })); // is correct