In JavaScript, reduce() is a built-in array method that allows you to iterate over an array and reduce it to a single value.

Here's the syntax for using reduce():

array.reduce(callback[, initialValue])

The reduce() method takes two parameters:

A callback function that is called on each element of the array.

An optional initial value that will be used as the initial value for the accumulator.

The callback function itself takes two parameters:

The accumulator - this is the value that is being accumulated as the reduce method iterates through the array.

The current value - this is the value of the current element being processed by the callback function.

Here's an example of how to use reduce() to find the sum of an array of numbers:

const numbers = [1, 2, 3, 4, 5];

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

console.log(sum); // Output: 15

In the example above, the reduce() method is called on the numbers array, and the callback function takes the accumulator and currentValue parameters. The function adds the accumulator and currentValue together on each iteration, and returns the new value of the accumulator.

The reduce() method can also be used to find the maximum or minimum value of an array, or to perform more complex operations on the array.