index.js

var a = Math.cos(Math.sin(((4 + 3) * 3)))
const PI = 3.14159
const GRAVITY = 9.8
// function concatenate(array) {
//     let a = ""
//     for (let i = 0; i < array.length; i++) {
//         const element=array[i]
//         a += element
//     }
//     return a
// }

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

// let sumFunction=function(a,b) {
//     return a+b
// }

// let arrowSum = (a,b) => { return a+b }

// (a,b) => a+b;
// (a,b) =>      console.log(a)
// console.log(b)
// return a+b

function onMyButtonClick() {
    //... do things
}

// myButton.addEventListener("click", onMyButtonClick)
// let action = "click"
// myButton.addEventListener(action, setButtonRed);    


function sumUpToFourNumbers(a,b=0,c=0,d=0) {
    return a +b +c+ d;
}

module.exports = sumUpToFourNumbers;

index.test.js

var sumUpToFourNumbers=require('./index.js')
test("Testing  template literals", ()=> {
    let person = {"name": "Ana", "address":"Elm St.", "city":"Gotham"}
    console.log("Hello " + person.name + " from " + person.address + " in " + person.city)
    console.log(`Hello ${person.name}\nfrom ${person.address}\nin ${person.city}`)
})

test("Default parameters",()=> {
    //given
    let firstNumber = 1
    //when
    let result=sumUpToFourNumbers(firstNumber)
    //then
    expect(result).toBe(firstNumber)
})

test("Default parameters 2",()=> {
    //given
    let firstNumber = 15
    let secondNumber = 89
    let expected = 104
    //when
    let result=sumUpToFourNumbers(firstNumber, secondNumber)
    //then
    expect(result).toBe(expected)
})