Here are the different approaches to append an array at the end of another array in JavaScript

push() Method with Spread Operator - Most Used

It uses push() combined with the spread operator to append elements of a2 to a1.

let a1 = [ 1, 2, 3 ];
let a2 = [ 4, 5, 6 ];
a1.push(...a2);
console.log(a1);

[ 1, 2, 3, 4, 5, 6 ]

Using concat() Method

It concatenates a1 and a2 into a new array, preserving the original arrays.

let a1 = [ 1, 2, 3 ];
let a2 = [ 4, 5, 6 ];
let result = a1.concat(a2);
console.log(result);

[ 1, 2, 3, 4, 5, 6 ]

Using Spread Operator

It combines a1 and a2 by spreading their elements into a new array.

let a1 = [1, 2, 3];
let a2 = [4, 5, 6];
let result = [...a1, ...a2];
console.log(result);

[ 1, 2, 3, 4, 5, 6 ]

Using for Loop

It iterates over a2 and appends each element to a1 using push().

let a1 = [1, 2, 3];
let a2 = [4, 5, 6];
for (let i = 0; i < a2.length; i++) {
    a1.push(a2[i]);
}
console.log(a1); 

[ 1, 2, 3, 4, 5, 6 ]

Using Array.prototype.reduce()

It uses reduce() to get elements from a2 into a copy of a1.

let a1 = [1, 2, 3];
let a2 = [4, 5, 6];
let result = a2.reduce((acc, val) => {
    acc.push(val);
    return acc;
}, [...a1]);
console.log(result); 

[ 1, 2, 3, 4, 5, 6 ]

Using splice() Method

It appends elements by modifying the original array. The elements of a2 are added at the end of a1 using splice().

let a1 = [1, 2, 3];
let a2 = [4, 5, 6];
a1.splice(a1.length, 0, ...a2);
console.log(a1);

[ 1, 2, 3, 4, 5, 6 ]

Using Array.from() Method

It creates a new array by joining existing ones. The elements of a1 and a2 are combined into a new array using Array.from() and the spread operator.

let a1 = [1, 2, 3];
let a2 = [4, 5, 6];
let result = Array.from([...a1, ...a2]);
console.log(result);

[ 1, 2, 3, 4, 5, 6 ]

Which Approach to Use?

• push() with Spread Operator: Use it for modifying the original array quickly and is efficient for appending smaller arrays.
• concat() Method: Recommended for immutability when you want to keep the original arrays unchanged.
• Spread Operator (...): Great for modern, readable, and concise code, especially when creating a new array.
• for Loop: Suitable when you need custom logic or condition checks during appending.
• reduce() Method: A more functional approach but may be less readable for simple tasks.
• splice() Method: Use when you want to modify the original array in-place without creating a new one.
• Array.from() Method: Provides a flexible way to create a new array when working with different iterable sources.