The filter function is failing to return the initial element of the array, contradicting what is displayed in the console.log output

When given two arrays of integers, I aim to create a new array containing only the unique values from both input arrays. There are many methods for achieving this, but I'm curious as to why my current approach is not producing the desired result.

const arr1 = [1, 2, 3, 4, 5];
const arr2 = [2, 3, 1, 0, 5];

// combining both arrays
const combined = arr1.concat(arr2);

// sorting the combined array
combined.sort();

// filtering out elements that are the same as the preceding element
const result = combined.filter((el, i, a) => {

  if ( (el != a[i-1]) ) {
    console.log("return", el);
    return el;
  }
});

// printing the output
console.log(result);

Despite zero being present in the combined array, the final output displays [1,2,3,4,5]. Upon using console.log, it's evident that the filter method is returning the element zero, yet it doesn't appear in the result array. What could be the issue?

Answer №1

Ensure that the lambda function within the filter method returns either a truthy or falsy value to operate correctly. Keep in mind that the value 0 is considered falsy and will be excluded from the result.

Consider utilizing a Set instead of a loop to eliminate duplicate values effectively.

const arr1 = [1, 2, 3, 4, 5];
const arr2 = [2, 3, 1, 0, 5];

// using Set
console.log(JSON.stringify([...new Set(arr1.concat(arr2))]));

// alternatively, filter with a lambda returning true/false
const result = arr1.concat(arr2).sort().filter((val, i, a) => val !== a[i-1]);
console.log(JSON.stringify(result));

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