sum function in Javascript

Question

When I create a simple Visual Studio project and add JavaScript and HTML items to it, I seem to have access to all built in functions, such as indexOf(), search(), slice(), range(), etc...

However, when I try to use the sum() function (e.g. console.log(sum(range(1, 10)));) I get the following error message:

Uncaught ReferenceError: sum is not defined

Putting this message within double quotes and adding the JavaScript keyword in Google did not bring up a webpage that tells me what I am missing, hence this basic question here.

Am I missing something like a library that sum is included in? What am I doing wrong that only this particular function is not recognized?

Answer 1

There is no built-in sum() function in Javascript.

You can create one easily with reduce():

function sum(arr) {
   return arr.reduce(function (a, b) {
      return a + b;
   }, 0);
}

var numbers = [1, 2, 3];

console.log(sum(numbers));

Or use reduce() as a oneliner:

var numbers = [1, 2, 3];

var totalSum = numbers.reduce(function (a, b) { return a + b; }, 0);

console.log(totalSum);

Answer 2

Just for the sake of it, here's a nice ES6 version:

const sum = (arr=[]) => arr.reduce((total, val) => total + val);

Answer 3

You can create prototype functions for arrays and if you do it in some file that executes before the usage of such functions, then they will basically work. For example:

console.log(typeof [].sum);

Array.prototype.sum = function() {
    let s = 0;
    for (let i = 0; i < this.length; i++) {
        s += this[i];
    }
    return s;
}

console.log(typeof [].sum);

console.log([2, 3, 5, 7, 11, 13, 17, 19, 23, 29].sum());

The reason for me saying this is that there is no sum. So you can create a range function that creates an array for you and call sum() on the resulting array.

    function range(start, end) {
        let output = [];
        while (start <= end) {
            output.push(start++);
        }
        return output
    }

    Array.prototype.sum = function() {
        let s = 0;
        for (let i = 0; i < this.length; i++) {
            s += this[i];
        }
        return s;
    }

console.log(range(1, 10).sum())

Answer 4

I think there is the potential to clean up the previous answers even further, take a look:

const sum = arr.reduce((accumulator, currentValue) => accumulator + currentValue);

The arrow function simplifies the function declaration syntax plus there is no need to declare initialValue since we want to just get the sum of the elements from the array of numbers.