How Can I Filter Properties in a JavaScript Object?

JavaScript filter() Method for Objects

While the Array type has the filter() method, the Object type does not. This article aims to provide a solution for implementing such a method.

Custom Object Filter Implementation

One approach involves extending the Object.prototype:

Object.prototype.filter = function(predicate) {
    var result = {};

    for (key in this) {
        if (this.hasOwnProperty(key) && !predicate(this[key])) {
            result[key] = this[key];
        }
    }

    return result;
};

Alternative Solutions

However, extending global prototypes is generally discouraged. Instead, consider these alternatives:

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), {});

2. Map and Spread Syntax

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

3. Object.assign

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

Example Usage

var foo = { bar: "Yes", moo: undefined };

var filtered = Object.filter(foo, property => typeof property === "undefined");

console.log(filtered); // { moo: undefined }

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