How to Implement a Custom `filter()` Method for Objects in JavaScript?

JavaScript: Implementing a Custom filter() Method for Objects

The ECMAScript 5 specification introduces the filter() prototype for arrays, but not for objects. Extending JavaScript's built-in objects is generally discouraged. However, if desired, one can create custom filter() functionality for objects using the following approaches:

1. Using reduce() and Object.keys()

Object.filter = (obj, predicate) =>
  Object.keys(obj)
    .filter(key => predicate(obj[key]))
    .reduce((res, key) => (res[key] = obj[key], res), {});

// Example:
const scores = {
  John: 2,
  Sarah: 3,
  Janet: 1
};
const filtered = Object.filter(scores, score => score > 1);
console.log(filtered);

2. Using reduce() and Object.keys() with Object.assign()

Object.filter = (obj, predicate) =>
  Object.keys(obj)
    .filter(key => predicate(obj[key]))
    .reduce((res, key) => Object.assign(res, { [key]: obj[key] }), {});

// Example: same as above

3. Using map() and spread syntax

Object.filter = (obj, predicate) =>
  Object.fromEntries(
    Object.entries(obj)
      .filter(([key, value]) => predicate(value))
      .map(([key, value]) => [key, value])
  );

// Example: same as above

4. Using Object.entries() and Object.fromEntries()

Object.filter = (obj, predicate) =>
  Object.fromEntries(
    Object.entries(obj).filter(([key, value]) => predicate(value))
  );

// Example: same as above

Remember, extending built-in prototypes can have unintended consequences. It's generally preferable to provide custom functions as stand-alone utilities or extend global objects specifically for specific functionality.

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