Implementing a Custom jest-like toBe() Matcher (Interview Question)

In a recent frontend interview, I was asked to implement a simplified version of Jest's toBe() matcher. The goal was to replicate the following behavior:

expect(5).toBe(5)        // true

expect(5).not.toBe(4)    // true

expect(5).toBe(10)       // false + error

This question is less about Jest itself and more about JavaScript fundamentals, especially closures, API design, and error handling.

function expect(received) {

  function toBe(expected, isNot = false) {

    const pass = received === expected;



    if (isNot ? !pass : pass) {

      return true;

    }



    throw new Error(

      isNot

        ? `Expected ${received} not to be ${expected}`

        : `Expected ${received} to be ${expected}`

    );

  }



  return {

    toBe(expected) {

      return toBe(expected);

    },

    not: {

      toBe(expected) {

        return toBe(expected, true);

      }

    }

  };

}

How It Works

Closures are used to retain the received value inside render

toBe() performs a strict equality check

.not simply flips the assertion logic

Errors are thrown to signal test failures, just like real test runners

Key Takeaway

Testing libraries like Jest are built on simple JavaScript principles. If you understand closures and function composition, you can easily reason about how these tools work internally - an essential skill for senior frontend developers.