const head = arr => (arr && arr.length ? arr[0] : undefined);

head([1, 2, 3]); // 1
head([]); // undefined
head(null); // undefined
head(undefined); // undefined

const primes = num => {
  let arr = Array.from({ length: num - 1 }).map((x, i) => i + 2),
    sqroot = Math.floor(Math.sqrt(num)),
    numsTillSqroot = Array.from({ length: sqroot - 1 }).map((x, i) => i + 2);
  numsTillSqroot.forEach(x => (arr = arr.filter(y => y % x !== 0 || y === x)));
  return arr;
};

primes(10); // [2, 3, 5, 7]

export const Main = () => {
    const [title, setTitle] = useState('');
    
    return (
        <ChildComponent
            title={title}
            onChange={e => setTitle(e.target.value)}
        />
    );
};

export const ChildComponent = ({ title, onChange }) => {
    return (
        <Box component="form" onSubmit={() => {}} noValidate mt={5}>
            <TextField
                fullWidth
                id="title"
                label="Title"
                name="title"
                autoComplete="title"
                autoFocus
                color="primary"
                variant="filled"
                InputLabelProps={{ shrink: true }}
                value={title}
                onChange={onChange}
            />
        </Box>
    );
};

const reverse = str => str.split('').reverse().join('');
reverse('hello world');     
// Result: 'dlrow olleh'

const express = require('express')
const app = express()
const port = 3000

app.get('/', (req, res) => {
  res.send('Hello World!')
})

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`)
})

const getURLParameters = url =>
(url.match(/([^?=&]+)(=([^&]*))/g) || []).reduce(
    (a, v) => (
      (a[v.slice(0, v.indexOf('='))] = v.slice(v.indexOf('=') + 1)), a
    ),
    {}
);