Implement custom Array.prototype.filter

Implement a custom myFilter function that works like the built-in Array.prototype.filter method.

Array.prototype.filter is a method that creates a new array with all elements that pass the test implemented by the provided function, e.g. filter all numbers greater than 3:

const numbers = [1, 2, 3, 4, 5];

const filtered = numbers.myFilter((number) => {
    return number > 3;
});

console.log(filtered); // Output: [4, 5]

Solution

To implement myFilter, we need to iterate over the array and call the callback function for each element to check if it passes the test. If the callback function returns true, we add the element to the new array.

Algorithm:

1. Initialize an empty array to store the result.
2. Iterate over the array.
3. For each element, call the callback function to check if it passes the test.
   • It runs the callback with arguments: (element, index, array)
   • If the callback function returns true, add the element to the result array.
4. Return the result array.

Implementation

Array.prototype.myFilter = function(callback) {
  const result = [];

  // `this` refers to the array on which myFilter is called
  for (let i = 0; i < this.length; i++) {
    // Check if the current element passes the test
    if (callback(this[i], i, this)) {
      result.push(this[i]);
    }
  }

  return result;
};

// Example of usage
const numbers = [1, 2, 3, 4, 5];
const filtered = numbers.myFilter((number) => {
  return number > 3;
});

console.log(filtered);  // Output: [4, 5]

Summary

• myFilter iterates through the array.
• For each element, it runs the callback with (element, index, array).
• If the callback returns true, the element is added to the result.
• It returns a new array, just like the built-in filter.