Created a lib folder for custom function

lib/Tank.ts

@@ -0,0 +1,38 @@
+import { setStats, isLive, isOther, critMultiplier } from "./helpers";
+
+export interface ITank {
+  name: string;
+  health: number;
+  attackDelay: number;
+}
+
+type BuildTank = (name: string) => ITank;
+export const buildTank: BuildTank = (name) => ({
+  name,
+  health: 100,
+  attackDelay: 0,
+} as ITank);
+
+type Attack = (_: ITank[], tank: ITank, tIndex: number, tanks: ITank[]) => ITank[];
+const attack: Attack = (_acc, tank, _iTank, tanks) => {
+  // [TODO]: Refactor this imperative block
+  if (tank.attackDelay === 0) {
+    const target = tanks.filter(isOther(tank.name))[Math.floor(Math.random() * tanks.length)];
+    const attackDamage = critMultiplier(tank.health) * tank.health / 100;
+    target.health -= attackDamage;
+  };
+
+  return tanks.map(setStats);
+};
+
+type Attacks = (tanks: ITank[]) => ITank[];
+const attacksRound: Attacks = (tanks) =>
+  tanks
+    .reduce(attack,  tanks)
+    .filter(isLive);
+
+type Battle = (remainingTanks: ITank[]) => ITank;
+export const battleItOut: Battle = (remainingTanks: ITank[]) =>
+  remainingTanks.length === 1
+    ? remainingTanks[0]
+    : battleItOut(attacksRound(remainingTanks));

lib/helpers.ts

@@ -0,0 +1,17 @@
+import { ITank } from "./Tank";
+
+type CritMultiplier = (health: number) => number;
+export const critMultiplier: CritMultiplier = (health) =>
+  Math.floor(Math.random() * 10) >= 10 - health / 10 ? 1 : 2;
+
+type SetStats = (tank: ITank) => ITank;
+export const setStats: SetStats = (tank) => ({
+  ...tank,
+  attackDelay: tank.attackDelay === 0 ? Math.floor(tank.health / 10) : tank.attackDelay - 1,
+} as ITank);
+
+type IsLive = (tank: ITank) => boolean;
+export const isLive: IsLive = (tank) => tank.health >= 0;
+
+type IsOther = (name: string) => (tank: ITank) => boolean;
+export const isOther: IsOther = (name) => (tank) => tank.name !== name;