How to Merge Arrays in JavaScript

Merging arrays in JavaScript is a common task and can be accomplished in several ways depending on your needs. Here are some of the most common methods:

1. Using the concat() Method#

The concat() method creates a new array by combining multiple arrays.

const array1 = [1, 2, 3];
const array2 = [4, 5, 6];

const mergedArray = array1.concat(array2);
console.log(mergedArray); // Output: [1, 2, 3, 4, 5, 6]

2. Using the Spread Operator (...)#

The spread operator is a more modern and concise way to merge arrays.

const array1 = [1, 2, 3];
const array2 = [4, 5, 6];

const mergedArray = [...array1, ...array2];
console.log(mergedArray); // Output: [1, 2, 3, 4, 5, 6]

3. Using Array.prototype.push() with the Spread Operator#

If you want to merge arrays into an existing array, you can use push() with the spread operator.

const array1 = [1, 2, 3];
const array2 = [4, 5, 6];

array1.push(...array2);
console.log(array1); // Output: [1, 2, 3, 4, 5, 6]

4. Using Array.prototype.unshift() with the Spread Operator#

Similarly, if you want to merge arrays at the beginning of an existing array, use unshift() with the spread operator.

const array1 = [1, 2, 3];
const array2 = [4, 5, 6];

array1.unshift(...array2);
console.log(array1); // Output: [4, 5, 6, 1, 2, 3]

5. Using Array.prototype.reduce()#

For more complex merging scenarios, such as merging multiple arrays dynamically, reduce() can be useful.

const arrays = [
    [1, 2],
    [3, 4],
    [5, 6],
];

const mergedArray = arrays.reduce((acc, curr) => acc.concat(curr), []);
console.log(mergedArray); // Output: [1, 2, 3, 4, 5, 6]

6. Using Array.prototype.flat()#

If you have nested arrays and want to merge them into a single array, flat() can help.

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

const mergedArray = nestedArrays.flat(2); // The argument specifies the depth to flatten
console.log(mergedArray); // Output: [1, 2, 3, 4, 5]

Each of these methods has its use cases, so you can choose the one that best fits your needs for merging arrays.