Answer to stackoverflow question 49156502

49156502.ts

function isDate(obj: any): obj is Date {
    return typeof obj === 'object' && 'toISOString' in obj;
}

function isString(obj: any): obj is string {
    return typeof obj === 'string';
}

interface JWT {
    id: string;
    auth_token: string;
    expires_in: Date; 
}

function isJwt(obj: any): obj is JWT {
    const o = obj as JWT;
    return o !== null
        && typeof o === 'object'
        && isString(o.id)
        && isString(o.auth_token)
        && isDate(o.expires_in);
}

function print(jwt: any) {
    if (typeof jwt === 'string') {
        try {
            jwt = JSON.parse(jwt);
        } catch (e) {
            console.error(`String is not JSON: ${jwt}`);
        }
    }
    if (isJwt(jwt)) {
        console.log(`Found jwt: ${jwt.id} ${jwt.auth_token} ${jwt.expires_in}`);
    } else {
        console.error(`Object is not of type jwt: ${jwt}`);
    }
}

print(42);
print('failing');
print(null);
print(undefined);
print({});
print({ id: 'id01', auth_token: 'token01', expires_in: new Date(2018, 11, 25) });