Javascript reduce() – why is this returning false?

April 8, 2022

Basically, I want to evaluate all items in an array. If every item is of equal type AND value (strict equality check) return true, otherwise return false.
What am I missing here? – Thanks!

function equalityChecker(arr) {
  return arr.reduce((prev, curr) => {
    if (prev !== curr) {
      return false;
    } 
    if (prev === curr) {
      return true;
    }
  });
}

console.log(equalityChecker([0,0,0,0,0,0])); //returns false
console.log(equalityChecker([1,1,1,1,1,1])); //returns false

>Solution :

Array#reduce returns a value, and you return the result of a comparison.

In the next iteration, you use this boolean value and checks againsta value from the array.

Hint 1: Reduce does not work for this approach.

The other diadvantage is the iteration length, it loops for all elements despite a false value.

Hint 2: You need a method which returns early.

Result: Take Array#every and compare each element with the first item.

function equalityChecker(array) {
    return array.every((value, _, [first]) => first === value);
}

console.log(equalityChecker([0, 0, 0, 0, 0, 0])); //  true
console.log(equalityChecker([1, 1, 1, 1, 1, 1])); //  true
console.log(equalityChecker([0, 0, 0, 1, 0, 0])); // false