type Opt = { id: string, name: string }

interface MultiProps {
    isMultiple: true;
    options: Opt[];
    id: string[];
    onChange: (id: string[]) => void;
}

interface SingleProps {
    isMultiple: false;
    options: Opt[];
    id: string;
    onChange: (id: string) => void;
}

type SelectProps = MultiProps | SingleProps;

function Select(props: SelectProps) {
    if (props.isMultiple) {
        // works
        const { id, onChange } = props;
        onChange(id);
    } else {
        // fail
        const { id, onChange } = props;
        onChange(id); // error here
    }
}

let m: MultiProps = {
    isMultiple: true,
    options: [{ id: 'a', name: 'A' }],
    id: ['a'],
    onChange: null
};

let s: SingleProps = {
    isMultiple: false,
    options: [{ id: 'a', name: 'A' }],
    id: 'a',
    onChange: null
};

Select(m);
Select(s);