The map() method can be used to modify all elements of an array.   Below, I have a function that will take an array of sales prices, and mark them up by 10% to account for sales tax.  On line 3, if I return the item without the parseFloat function, it converts all the elements in the array to a string, so I use  the parseFloat function (to return a number with decimals) and the toFixed() method round to two decimal places.

function addTax(item) {
    const taxRate = .10;
    return parseFloat((item * (1 + taxRate)).toFixed(2));
}

const sales=[1,20,25,5,30,100];
const output = sales.map(addTax);
console.log(output);
// output = [ 1.1, 22, 27.5, 5.5, 33, 110 ]

Next, I will modify an array of objects using the map() method.  The addPrefix function will look at the sex of the item.  If the sex is male, it will add “Mr. ” to the name, and if the sex is female, it will add “Ms. ” to the name.

const names = [
    {
      name: 'Jones',
      sex: 'male'
    },{
      name: 'Smith',
      sex: 'female'
    },{
      name: 'Tanaka',
      sex: 'male'}
];

function addPrefix(item) {
    if(item.sex === 'male') {
        item.name = "Mr. " + item.name;
    } else if (item.sex === 'female') {
        item.name = "Ms. " + item.name;
    }
    return item;
}

const output = names.map(addPrefix);
console.log(output); // output = 
[ { name: 'Mr. Jones', sex: 'male' },
  { name: 'Ms. Smith', sex: 'female' },
  { name: 'Mr. Tanaka', sex: 'male' } ]