5.3 Everything (Eloquent JavaScript Solutions)

5-3 Eloquent Javascript Solutions.js

// 5.3 Everything
//every using a loop
function every(array, test) {
  for (let element of array) {
    if (test(element) === false) {
      return false;
    }
  }
  return true;
}

console.log(every([1, 3, 5], n => n < 10));
// → true
console.log(every([2, 4, 16], n => n < 10));
// → false
console.log(every([], n => n < 10));
// → true

// every using array.some
function every(array, test) {
  return array.some(test);
}

console.log(every([1, 3, 5], n => n < 10));
// → true
console.log(every([2, 4, 16], n => n < 10));
// → false
console.log(every([], n => n < 10));
// → true

Hi Jim,

just merely applying the some() method will not work, since latter returns true when one element in the array passes the test.

To test if ALL elements pass the test with the some() method, use De Morgan’s laws, which states that
!(A && B) === !A || !B

We can modify De Morgan’s laws by using a double negation:
(A && B) === ! (!A || !B)

Here the solution:

function every(array, test) {
    return !array.some(element => !test(element));
}

Hi Jim,

just merely applying the some() method will not work, since latter returns true when one element in the array passes the test.

To test if ALL elements pass the test with the some() method, use De Morgan’s laws, which states that !(A && B) === !A || !B

We can modify De Morgan’s laws by using a double negation: (A && B) === ! (!A || !B)

Here the solution:

function every(array, test) {
    return !array.some(element => !test(element));
}

I used the some method with for/of loop, but now I understand how to use some to confirm that all the items of the list passed the test with the help of De Morgan's law.
Thanks, Christoph!