Shift last given number of elements to front of array JavaScript

In JavaScript, moving elements from the end of an array to the front is a common array manipulation task. This tutorial shows how to create a function that shifts the last n elements to the beginning of an array in-place.

Problem Statement

We need to create an array method reshuffle() that:

• Takes a number n (where n ? array length)
• Moves the last n elements to the front
• Modifies the array in-place
• Returns true on success, false if n exceeds array length

Example Input and Output

// Original array
const arr = ["blue", "red", "green", "orange", "yellow", "magenta", "cyan"];

// After reshuffle(3), last 3 elements move to front:
// Result: ["yellow", "magenta", "cyan", "blue", "red", "green", "orange"]

Implementation

Here's how to implement the reshuffle() method using splice() and unshift():

const arr = ["blue", "red", "green", "orange", "yellow", "magenta", "cyan"];

Array.prototype.reshuffle = function(num) {
    const { length: len } = this;
    
    // Check if num exceeds array length
    if (num > len) {
        return false;
    }
    
    // Extract last 'num' elements
    const deleted = this.splice(len - num, num);
    
    // Add them to the front
    this.unshift(...deleted);
    
    return true;
};

console.log("Original array:", arr);
console.log("Reshuffle 4 elements:", arr.reshuffle(4));
console.log("Modified array:", arr);

Original array: [ 'blue', 'red', 'green', 'orange', 'yellow', 'magenta', 'cyan' ]
Reshuffle 4 elements: true
Modified array: [ 'orange', 'yellow', 'magenta', 'cyan', 'blue', 'red', 'green' ]

How It Works

1. Validation: Check if num exceeds array length
2. Extract: splice(len - num, num) removes last n elements and returns them
3. Prepend: unshift(...deleted) adds extracted elements to the front
4. Return: Boolean indicating success/failure

Alternative Implementation

Using array slicing and concatenation:

Array.prototype.reshuffleAlternative = function(num) {
    if (num > this.length) return false;
    
    const lastElements = this.slice(-num);
    const remainingElements = this.slice(0, -num);
    
    // Clear array and rebuild
    this.length = 0;
    this.push(...lastElements, ...remainingElements);
    
    return true;
};

const arr2 = ["a", "b", "c", "d", "e"];
console.log("Success:", arr2.reshuffleAlternative(2));
console.log("Result:", arr2);

Success: true
Result: [ 'd', 'e', 'a', 'b', 'c' ]

Edge Cases

const testArray = [1, 2, 3];

// Test edge cases
console.log("Empty reshuffle:", testArray.reshuffle(0)); // Should work
console.log("Array after 0:", testArray);

console.log("Exceeds length:", testArray.reshuffle(5)); // Should fail
console.log("Array unchanged:", testArray);

Empty reshuffle: true
Array after 0: [ 1, 2, 3 ]
Exceeds length: false
Array unchanged: [ 1, 2, 3 ]

Conclusion

The reshuffle() method efficiently moves array elements using splice() and unshift(). It handles edge cases and modifies arrays in-place while returning a success indicator.