interface State {
    username: string;
    password: string;
}

class LoginForm extends React.Component<Props, State> {
    readonly state: Readonly<State> = {
        username: '',
        password: ''
    }

    onChange = (key: keyof State) => (e: React.FormEvent<HTMLInputElement>) => {
        const value = e.currentTarget.value;
        this.setState({ [key]: value });  // error
    }

    render() {
        return (
            ...
            <TextField
                ...
                onChange={this.onChange('username')}
            />
            <TextField
                ...
                onChange={this.onChange('password')}
            />
            ...
        )
    }
}

onChange = (key: 'username' | 'password') => ...