filterObject

Keeps the own enumerable properties that satisfy a predicate.

Code

filterObject.ts

/** Converts TypeScript numeric keys to the strings received at runtime. */
export type FilterObjectKey<T extends object> =
  Extract<keyof T, PropertyKey> extends infer Key
    ? Key extends number
      ? number extends Key
        ? string
        : `${Key}`
      : Key
    : never;

const readDescriptorValue = (
  descriptor: PropertyDescriptor,
  receiver: object
): unknown => {
  if (Object.hasOwn(descriptor, "value")) return descriptor.value;
  return descriptor.get
    ? Reflect.apply(descriptor.get, receiver, [])
    : undefined;
};

/** Keeps the own enumerable properties that satisfy a predicate. */
export const filterObject = <T extends object>(
  object: T extends readonly unknown[] ? never : T,
  predicate: (
    this: void,
    value: T[keyof T],
    key: FilterObjectKey<T>,
    object: T
  ) => boolean
): Partial<T> => {
  if (Array.isArray(object)) {
    throw new TypeError("object must not be an array");
  }
  const filtered = Object.create(null) as Partial<T>;

  const properties = Reflect.ownKeys(object).flatMap((key) => {
    const descriptor = Reflect.getOwnPropertyDescriptor(object, key);
    return descriptor ? [{ key, descriptor }] : [];
  });
  const entries = properties
    .filter(({ descriptor }) => descriptor.enumerable)
    .map(({ key, descriptor }) => ({
      key,
      value: readDescriptorValue(descriptor, object) as T[keyof T],
    }));

  for (const { key, value } of entries) {
    if (predicate(value, key as FilterObjectKey<T>, object)) {
      Object.defineProperty(filtered, key, {
        value,
        enumerable: true,
        configurable: true,
        writable: true,
      });
    }
  }

  return filtered;
};

Download

wget

Terminal

wget -O src/lib/filterObject.ts https://raw.githubusercontent.com/jrTilak/lazykit/HEAD/registry/functions/filterObject.ts

degit

Terminal

temp_dir="$(mktemp -d)" && bunx degit jrTilak/lazykit/registry/functions "$temp_dir" && mv "$temp_dir/filterObject.ts" src/lib/filterObject.ts && rm -r "$temp_dir"

Examples

filterObject.example.ts

import { filterObject } from "./filterObject";

const user = { name: "Charlie", age: 28, active: true, role: "editor" };
const textFields = filterObject(user, (value) => typeof value === "string");

console.log(textFields);
// { name: "Charlie", role: "editor" }

Props

• object (T) — Non-array object whose own enumerable properties should be tested.
• predicate ((value, key, object) => boolean) — Returns true for properties to retain and is invoked without a receiver. Numeric TypeScript keys arrive as their runtime string form.

Returns

Partial<T> — A null-prototype object containing the matching string and symbol properties, so inherited names cannot appear as unprocessed values. Arrays are rejected with TypeError because the result is a plain object. Special keys such as __proto__ are copied as safe own data properties.