## Using `Array.prototype.reduce()`

Introduced in ES5, the `reduce()`

method can help reduce an array to a single value (by applying a function against an accumulator and each value of the array). To sum an array of integers using `reduce()`

, you can do the following:

// ES5+ const integers = [-1, 2, 3, -4, 5]; const sum = integers.reduce(function (accumulator, value) { return accumulator + value; }, 0); console.log(sum); // output: 5

The same code can be written in a shorter syntax using the arrow function (introduced in ES6). For example:

// ES6+ const integers = [-1, 2, 3, -4, 5]; const sum = integers.reduce((accumulator, value) => accumulator + value, 0); console.log(sum); // output: 5

Note that the `0`

at the end is the initial value, and the `accumulator`

holds the "reduced" value returned by the function in each iteration.

You may also use the `Array.prototype.reduceRight()`

method. The difference between `reduce()`

and `reduceRight()`

is that the former goes through values left-to-right while the latter goes through values from right-to-left.

## Using a Loop

You can simply use a loop to iterate over the entire array and sum the values. For example, to do that using a `for`

loop, we can do the following:

const integers = [-1, 2, 3, -4, 5]; let sum = 0; for (let i=0; i < integers.length; i++) { sum += integers[i]; } console.log(sum); // output: 5

You can also refactor the `for`

loop in a shorter syntax like so:

const integers = [-1, 2, 3, -4, 5]; let sum = 0; for (let i = 0; i < integers.length; sum += integers[i++]); console.log(sum); // output: 5

Similarly, you may use other loops to sum all elements of an array of integers.

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