How can I effectively filter the key-value pairs in a JavaScript object?

JavaScript: filter() for Objects

The filter() prototype exists for Array types in ECMAScript 5, but not for Object types. To implement a filter() function for Objects in JavaScript, you can use the following strategies:

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

This method utilizes reduce() to loop through the object's keys and Object.keys() to retrieve those keys. Then, it filters the keys based on a predicate function and returns a new object with the filtered key-value pairs:

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

2. Using map() and Spread Syntax

A similar approach to (1), but instead of reduce(), you can use map() and the spread syntax (...) to create a new object:

Object.filter = (obj, predicate) => {
  Object.keys(obj)
    .filter(key => predicate(obj[key]))
    .map(key => ({ [key]: obj[key] }))
    .reduce((a, b) => ({ ...a, ...b }), {});
};

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

This method uses Object.entries() to convert the object into an array of key-value pairs and then Object.fromEntries() to reconstruct the object after filtering:

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

4. Caveat: Avoid Extending Object.prototype

While these solutions provide functional filter() implementations for Objects, it's considered bad practice to extend Object.prototype directly. Instead, provide your filter function as a stand-alone utility function, similar to existing JavaScript global utility functions like Object.assign() and Object.keys().

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