How to use reduce function in js

The reduce function is used for accumulation operations to obtain a single value: receiving an array, a callback function and an initial value (optional). The callback function handles the accumulator (which stores the accumulated results) and the current element. The initial value is the starting value of the accumulator, which defaults to the first element of the array. Use cases include summing, averaging, concatenating arrays, filtering, and grouping.

Usage of reduce function in JS

The reduce function is a function in JavaScript, used to manipulate an array The elements in are accumulated and finally a single value is obtained. Its usage is as follows:

<code class="javascript">const result = array.reduce(callback, initialValue);</code>

Where:

• array: The array to be accumulated.
• callback: Callback function for accumulation operation.
• initialValue: The accumulated initial value (optional).

Callback function

The callback function receives two parameters:

• accumulator: Accumulator, Stores the current accumulated results.
• currentValue: The array element currently being processed.

initialValue

initialValue is the initial value of the accumulator. If not specified, the first element of the array will be used as the initial value.

Usage

The reduce function is often used in the following scenarios:

• Sum: Calculate all elements in the array Sum.
• Average: Calculate the average of all elements in the array.
• Concatenate arrays: Concatenate all elements in the array into a string.
• Filter array: Filter elements in the array based on conditions.
• Group array: Group the elements in an array based on a specific key.

Example

Sum:

<code class="javascript">const numbers = [1, 2, 3, 4, 5];
const sum = numbers.reduce((accumulator, currentValue) => accumulator + currentValue, 0);
console.log(sum); // 输出：15</code>

Average:

<code class="javascript">const numbers = [1, 2, 3, 4, 5];
const average = numbers.reduce((accumulator, currentValue) => accumulator + currentValue, 0) / numbers.length;
console.log(average); // 输出：3</code>

Connection array:

<code class="javascript">const names = ['John', 'Mary', 'Bob'];
const joinedString = names.reduce((accumulator, currentValue) => accumulator + ', ' + currentValue);
console.log(joinedString); // 输出：John, Mary, Bob</code>

Filter array:

<code class="javascript">const numbers = [1, 2, 3, 4, 5];
const evenNumbers = numbers.reduce((accumulator, currentValue) => {
  if (currentValue % 2 === 0) {
    accumulator.push(currentValue);
  }
  return accumulator;
}, []);
console.log(evenNumbers); // 输出：[2, 4]</code>

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