Javascript reduce() - why is this returning false?-CodePudding

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

CodePudding user response：

reduce compares the result of the previous test with the current index.

It doesn't compare the previous index with the current index.

(The exception is if you don't pass a value as the second argument to reduce, in which case the first comparison is between the first two indexes).

The first test compares 0 and 0 so the second test compares true and 0, so the third test compares false and false and so on.

When you debugged this you should have added logging which would have shown you what was going on.

function equalityChecker(arr) {
  console.log(arr);
  return arr.reduce((prev, curr) => {
    console.log(`prev: ${prev}, curr: ${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

CodePudding user response：

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 disadvantage is the iteration length, it loops for all elements despite of a false result.

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