How to split last n digits of each value in the array with JavaScript?

We have an array of mixed values like numbers and strings, and we want to extract the last n digits from each element that has enough characters.

const arr = ["", 20191219, 20191220, 20191221, 20191222, 20191223, 20191224, 20191225];

We need to write a JavaScript function that takes this array and a number n. If an element contains more than or equal to n characters, the function should return only the last n characters. Otherwise, the element should remain unchanged.

Solution

Here's how we can implement this function using the map() method and string manipulation:

const arr = ["", 20191219, 20191220, 20191221, 20191222, 20191223, 20191224, 20191225];

const splitElement = (arr, num) => {
    return arr.map(el => {
        if (String(el).length <= num) {
            return el;
        }
        const part = String(el).substr(String(el).length - num, num);
        return +part || part;
    });
};

console.log(splitElement(arr, 2));
console.log(splitElement(arr, 1));
console.log(splitElement(arr, 4));

[
    '', 19, 20, 21,
    22, 23, 24, 25
]
[
    '', 9, '0', 1,
    2, 3, 4, 5
]
[
    '', 1219, 1220,
    1221, 1222, 1223,
    1224, 1225
]

How It Works

The function works by:

• Converting to string: String(el) converts each element to a string for uniform processing
• Length check: If the string length is less than or equal to n, return the original element
• Extracting substring: substr() extracts the last n characters starting from length - num position
• Type conversion: +part || part converts numeric strings back to numbers, but keeps non-numeric strings as strings

Alternative Using slice()

You can also use the modern slice() method instead of the deprecated substr():

const splitElementModern = (arr, num) => {
    return arr.map(el => {
        const str = String(el);
        if (str.length <= num) {
            return el;
        }
        const part = str.slice(-num);
        return +part || part;
    });
};

const testArr = ["", 20191219, 20191220, 20191221];
console.log(splitElementModern(testArr, 3));

[ '', 219, 220, 221 ]

Conclusion

This function efficiently extracts the last n digits from array elements using string manipulation and the map method. The slice(-num) approach is preferred over substr() for modern JavaScript development.