Let’s look at some ways to easily add 1 year to a Date object in JavaScript.
1. Date setFullYear() and getFullYear() Methods
To add 1 year to a date, call the getFullYear() method on the date to get the year, then call the setFullYear() method on the date, passing the sum of getFullYear() and 1 as an argument, i.e., date.setFullYear(date.getFullYear() + 1).
For example:
function addOneYear(date) {
date.setFullYear(date.getFullYear() + 1);
return date;
}
// April 20, 2022
const date = new Date('2022-04-20T00:00:00.000Z');
const newDate = addOneYear(date);
// April 20, 2023
console.log(newDate); // 2023-04-20T00:00:00.000Z
Our addOneYear() function takes a Date object and returns the same Date with the year increased by 1.
The Date getFullYear() method returns a number that represents the year of a particular date.
The Date setFullYear() method sets the year of a date to a specified number.
Avoiding Side Effects
The setFullYear() method mutates the Date object it is called on. This introduces a side effect into our addOneYear() function. To avoid modifying the passed Date and create a pure function, make a copy of the Date and call setFullYear() on this copy, instead of the original.
function addOneYear(date) {
// Making a copy with the Date() constructor
const dateCopy = new Date(date);
dateCopy.setFullYear(dateCopy.getFullYear() + 1);
return dateCopy;
}
// April 20, 2022
const date = new Date('2022-04-20T00:00:00.000Z');
const newDate = addOneYear(date);
// April 20, 2023
console.log(newDate); // 2023-04-20T00:00:00.000Z
// Original not modified
console.log(date); // 2022-04-20T00:00:00.000Z
Functions that don’t modify external state (i.e., pure functions) tend to be more predictable and easier to reason about. This makes it a good practice to limit the number of side effects in your programs.
2. date-fns addYears() Function
Alternatively, we can use the pure addYears() function from the date-fns NPM package to quickly add 1 year to a date.
import { addYears } from 'date-fns';
// April 20, 2022
const date = new Date('2022-04-20T00:00:00.000Z');
const newDate = addYears(date, 1);
// April 20, 2023
console.log(newDate); // 2023-04-20T00:00:00.000Z
// Original not modified
console.log(date); // 2022-04-20T00:00:00.000Z
This function takes a Date object and the number of years to add as arguments, and returns a new Date object with the newly added years. It does not modify the original date passed to it.
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