calc.js

function sum(a, b) {
  return a + b;
}

function divide(a, b) {
  return a / b;
}

function calculatePercentage(a, b) {
  return divide(a, b) * 100;
}


module.exports = {
  sum,
  divide,
  calculatePercentage
}

calc.test.js

const {
    sum,
    divide,
    calculatePercentage
} = require("./calc"); // import the function

// This function tests the sum() function
// and prints a message to the console
// depending on whether the test passed or failed.
function testSum() { // This is the unit test
    // because it tests a single unit of code.
    const result = sum(7, 3);
    const expected = 10;
    if(result !== expected) {
        console.log("TEST FAILED x_x");
    }else {
        console.log("TEST PASSED :)");
    }
}

function testPercentage() {
    // This is the integration test
    // because it tests the integration of
    // the divide() and calculatePercentage()
    // functions.
    const result = calculatePercentage(5, 20);
    const expected = 25;
    if(result !== expected) {
        console.log("TEST FAILED x_x");
    }else {
        console.log("TEST PASSED :)");
    }
}
testSum();
testPercentage();

https://jestjs.io/