Javascript – AssertionError: expected [Function] to throw an error

I’m experiencing an inconsistency between the behavior of my unit tests and the actual functionality of a JavaScript function, ConvertHandler.getNum, within a unit conversion application.

Problem Description:

My unit test convertHandler.getNum should return an error on a double-fraction input is designed to assert that the getNum function throws an error with the message "invalid number" when provided with an input like "3/2/3 mi", which contains a double fraction. However, while running the test suite, the error is not being thrown as expected.

convertHandler should correctly return an error on a double-fraction (i.e. 3/2/3).

So, when I manually input "3/2/3 mi" in the browser frontend, the application does display the correct error message "invalid number". This discrepancy between the behavior of the unit test and the frontend interaction raises questions about the accuracy of my unit tests or potential issues with the environment.

Why the unit test fails to throw the expected error while the frontend interaction produces the correct error message ?

controllersconvertHandler.js:

function regexSplit(input) {
  const matches = input.match(/([.d/]+)|([a-zA-Z]+)/g);

  const number = matches
    ? matches.find((match) => /[.d/]+/.test(match))
    : "1";
  const string = matches
    ? matches.find((match) => /[a-zA-Z]+/.test(match))
    : "";

  return [number, string];
}

function onlyOneDiv(input) {
  let fractionParts = input.split("/");
  if (fractionParts.length > 2) {
    return false;
  }
  return fractionParts;
}

function ConvertHandler() {
  this.getNum = function (input) {
    const number = regexSplit(input)[0] || "1"; // if nothing is provided it will default to 1.

    const fractionArray = onlyOneDiv(number);

    if (isNaN(parseFloat(fractionArray[0])) || isNaN(parseFloat(fractionArray[1] || "1"))) {
      return "invalid number";
    }

    return parseFloat(fractionArray[0]) / parseFloat(fractionArray[1] || "1");
  };
}

module.exports = ConvertHandler;

tests1_unit-tests.js:

const chai = require("chai");
let assert = chai.assert;
const ConvertHandler = require("../controllers/convertHandler.js");
let convertHandler = new ConvertHandler();

suite("Unit Tests", function () {
  test("convertHandler.getNum should return an error on a double-fraction input", function () {
    assert.throw(() => convertHandler.getNum("3/2/3 mi"), "invalid number");
  });
});

Running test returns error in terminal:

convertHandler.getNum should return an error on a double-fraction input:

AssertionError: expected [Function] to throw an error

In browser, code displays the error "invalid number" when i enter the value: "3/2/3 mi" :

Update: If i use assert.strictEqual or assert.equal instead of asset.throw, the unit test throws error:

AssertionError: expected [Function] to equal ‘invalid number’