Add useConstructor hook (#11)

Introduces a `useConstructor` hook that executes a provided callback
only once during the component's lifecycle, similar to a constructor.

This ensures that initialization logic is performed only once, even
after re-renders or component remounts. Includes thorough tests to
confirm the hook's behavior in different scenarios.

Closes #7

Author: drebrez

CHANGELOG.md

@@ -6,3 +6,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+
+### Added
+
+- `useConstructor` hook that executes a provided callback only once during the component's lifecycle, similar to a constructor.

cypress/component/useConstructor.cy.tsx

@@ -0,0 +1,65 @@
+import React, { useState } from "react";
+import { useConstructor } from "../../src/lib/hooks/useConstructor";
+
+describe("useConstructor", () => {
+  it("calls the callback exactly once per mount, even after re-renders", () => {
+    const initSpy = cy.spy().as("initSpy");
+
+    const TestComponent: React.FC = () => {
+      const [count, setCount] = useState(0);
+      useConstructor(() => {
+        initSpy();
+      });
+
+      return (
+        <div>
+          <span data-cy="count">{count}</span>
+          <button onClick={() => setCount((c) => c + 1)}>Increment</button>
+        </div>
+      );
+    };
+
+    // Turn off StrictMode as it will double‑invoke mount logic
+    cy.mount(<TestComponent />, { strict: false });
+
+    cy.get("@initSpy").should("have.been.calledOnce");
+
+    // Cause several re-renders
+    cy.contains("Increment").click().click().click();
+    cy.get("[data-cy='count']").should("have.text", "3");
+
+    cy.get("@initSpy").should("have.been.calledOnce");
+  });
+
+  it("runs again after an unmount/remount (new instance)", () => {
+    const initSpy = cy.spy().as("initSpy");
+
+    const Wrapper: React.FC = () => {
+      const [show, setShow] = useState(true);
+      return (
+        <div>
+          <button onClick={() => setShow((s) => !s)}>Toggle</button>
+          {show && <Child />}
+        </div>
+      );
+    };
+
+    const Child: React.FC = () => {
+      useConstructor(() => initSpy());
+      return <div data-cy="child">Child</div>;
+    };
+
+    // Turn off StrictMode as it will double‑invoke mount logic
+    cy.mount(<Wrapper />, { strict: false });
+
+    cy.get("@initSpy").should("have.been.calledOnce");
+
+    cy.contains("Toggle").click(); // unmount
+    cy.get("[data-cy='child']").should("not.exist");
+
+    cy.contains("Toggle").click(); // remount
+    cy.get("[data-cy='child']").should("exist");
+
+    cy.get("@initSpy").should("have.callCount", 2);
+  });
+});

src/index.ts

@@ -1 +1,2 @@
+export * from "./lib/hooks/useConstructor";
 export * from "./lib/hooks/useHelloWorld";

src/lib/hooks/useConstructor.ts

@@ -0,0 +1,15 @@
+import { useRef } from "react";
+
+/**
+ * Custom hook that runs the provided callback only once during the component's lifecycle.
+ * @param callback - Function to be executed once on component mount.
+ */
+const useConstructor = (callback: () => void) => {
+  const calledRef = useRef(false);
+  if (!calledRef.current) {
+    callback();
+    calledRef.current = true;
+  }
+};
+
+export { useConstructor };