diff --git a/src/array.test.ts b/src/array.test.ts
index ef6137d..be9fd46 100644
--- a/src/array.test.ts
+++ b/src/array.test.ts
@@ -109,6 +109,20 @@ describe("array", () => {
     });
   });
 
+  describe("uniqueBy", () => {
+    it("removes duplicates", () => {
+      const arr = [{ foo: 1 }, { foo: 2 }, { foo: 1 }];
+      const result = arr.uniqueBy((obj) => obj.foo);
+      expect(result).toEqual([{ foo: 1 }, { foo: 2 }]);
+    });
+
+    it("uses the first object if there are multiple matching elements in the array", () => {
+      const arr = [{ foo: 1, bar: 2 }, { foo: 2 }, { foo: 1, baz: 3 }];
+      const result = arr.uniqueBy((obj) => obj.foo);
+      expect(result).toEqual([{ foo: 1, bar: 2 }, { foo: 2 }]);
+    });
+  });
+
   describe("everyHasSame", () => {
     it("returns true if all elements match", () => {
       // given everything has the same 'name' property
diff --git a/src/array.ts b/src/array.ts
index d67961f..1ce52aa 100644
--- a/src/array.ts
+++ b/src/array.ts
@@ -4,6 +4,7 @@ declare global {
   interface Array<T> {
     unique(): Array<T>;
     uniqueByKey(key: keyof T): Array<T>;
+    uniqueBy(f: (el: T, index: number, array: T[]) => PropertyKey): Array<T>;
     compact(): Array<NonNullable<T>>;
     isEmpty: boolean;
     nonEmpty: boolean;
@@ -31,6 +32,7 @@ declare global {
     asyncFilter(predicate: (v: T) => Promise<boolean>): Promise<Array<T>>;
     asyncSome(predicate: (v: T) => boolean | Promise<boolean>): Promise<boolean>;
     asyncMap<V>(f: (el: T, index: number, array: T[]) => Promise<V>): Promise<V[]>;
+    asyncFlatMap<V>(f: (el: T, index: number, array: T[]) => Promise<V | V[]>): Promise<V[]>;
     asyncForEach(f: (el: T, index: number, array: T[]) => Promise<any>): Promise<void>;
     sum(this: Array<number | undefined>): number;
     sum(f: (el: T, index: number, array: T[]) => number | undefined): number;
@@ -74,6 +76,9 @@ declare global {
   }
 
   interface ReadonlyArray<T> {
+    unique(): Array<T>;
+    uniqueByKey(key: keyof T): Array<T>;
+    uniqueBy(f: (el: T, index: number, array: T[]) => PropertyKey): Array<T>;
     compact(): Array<NonNullable<T>>;
     isEmpty: boolean;
     nonEmpty: boolean;
@@ -100,6 +105,7 @@ declare global {
     asyncFilter(predicate: (v: T) => Promise<boolean>): Promise<Array<T>>;
     asyncSome(predicate: (v: T) => boolean | Promise<boolean>): Promise<boolean>;
     asyncMap<V>(f: (el: T, index: number, array: T[]) => Promise<V>): Promise<V[]>;
+    asyncFlatMap<V>(f: (el: T, index: number, array: T[]) => Promise<V | V[]>): Promise<V[]>;
     asyncForEach(f: (el: T, index: number, array: T[]) => Promise<any>): Promise<void>;
     sum(this: ReadonlyArray<number | undefined>): number;
     sum(f: (el: T, index: number, array: ReadonlyArray<T>) => number | undefined): number;
@@ -158,15 +164,23 @@ Array.prototype.unique = function () {
 };
 
 /** Would be cool to allow an array of keys to make the criteria of "unique" more flexible */
-Array.prototype.uniqueByKey = function <T>(key: keyof T): T[] {
+Array.prototype.uniqueBy = function <T>(f: (el: T, index: number, array: T[]) => PropertyKey): T[] {
   const result: T[] = [];
-  const group = this.groupBy((item) => item[key]);
-  Object.keys(group).forEach((gKey) => {
-    result.push(group[gKey].first);
-  });
+  const set = new Set<PropertyKey>();
+  for (let i = 0; i < this.length; i++) {
+    const key = f(this[i], i, this);
+    if (!set.has(key)) {
+      result.push(this[i]);
+      set.add(key);
+    }
+  }
   return result;
 };
 
+Array.prototype.uniqueByKey = function <T>(key: keyof T): T[] {
+  return this.uniqueBy((el) => el[key]);
+};
+
 Array.prototype.compact = function () {
   return this.filter(isDefined);
 };
@@ -236,6 +250,13 @@ Array.prototype.asyncMap = async function <T, V>(
   return Promise.all(this.map(f));
 };
 
+Array.prototype.asyncFlatMap = async function <T, V>(
+  this: Array<T>,
+  f: (el: T, index: number, array: T[]) => Promise<V | V[]>,
+): Promise<V[]> {
+  return Promise.all(this.map(f)).then((result) => result.flat(1) as V[]);
+};
+
 Array.prototype.asyncForEach = async function <T>(
   this: Array<T>,
   f: (el: T, index: number, array: T[]) => Promise<any>,