object-assign-defined/index.d.ts

A simple module that works exactly as Object.assign, but skips assigning undefined values.

type NonUndefinedKeys<T> = {
  [K in keyof T]-?: undefined extends T[K] ? never : K;
}[keyof T];

type DefinedProps<T> = Pick<T, NonUndefinedKeys<T>>;

type MergeDefined<
  TTarget,
  TSources extends readonly any[]
> = TSources extends readonly [infer H, ...infer R]
  ? MergeDefined<
      TTarget & (H extends object ? DefinedProps<NonNullable<H>> : unknown),
      R extends readonly any[] ? R : []
    >
  : TTarget;

/**
 * Works like Object.assign, but skips properties whose value is `undefined` in the source objects.
 * Mutates and returns the target object/array.
 *
 * Type behavior:
 * - Only keys from sources whose value type does NOT include `undefined` are merged into the result type.
 * - `null` sources are ignored.
 * - Optional properties (which are `T[K] | undefined`) are excluded from the merged type.
 *
 * Example:
 *   objectAssignDefined({ a: 1 }, { b: 2 }, { c: undefined }) // => { a: 1, b: 2 }
 *   // type: { a: number } & { b: number }   (no "c")
 */
declare function objectAssignDefined<
  TTarget extends object,
  const TSources extends readonly any[]
>(target: TTarget, ...sources: TSources): MergeDefined<TTarget, TSources>;

export = objectAssignDefined;