How to Zip Two Arrays in JavaScript

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In programming, “zipping” refers to the process of combining two arrays into a single array of pairs. Each pair consists of elements from the same position in the original arrays. This operation is useful in various scenarios, such as when you need to combine related data from separate arrays for further processing or display. JavaScript, despite its vast array of built-in methods for array manipulation, does not include a native “zip” function. However, implementing one is straightforward. This article explores several ways to zip two arrays in JavaScript, ranging from basic loop-based methods to more advanced functional programming techniques.

Table of Contents

Basic Loop-Based Method

The simplest way to zip two arrays is by using a loop to iterate over the elements of the arrays, combining them into pairs. This method works well when both arrays are of the same length:

function zipArrays(array1, array2) {
    let zippedArray = [];
    for (let i = 0; i < array1.length; i++) {
        zippedArray.push([array1[i], array2[i]]);
    }
    return zippedArray;
}

// Example usage
const names = ['Alice', 'Bob', 'Charlie'];
const ages = [25, 30, 35];
console.log(zipArrays(names, ages));
// Output: [['Alice', 25], ['Bob', 30], ['Charlie', 35]]

This method assumes that both array1 and array2 have the same length. If they don’t, the loop will terminate at the end of the shortest array, and any remaining elements in the longer array will be ignored.

Using `Array.prototype.map()`

If you’re sure that both arrays have the same length, you can use the map() method for a more functional approach. The map() method creates a new array populated with the results of calling a provided function on every element in the calling array:

function zipArrays(array1, array2) {
    return array1.map((element, index) => [element, array2[index]]);
}

// Example usage remains the same

This method is concise and expressive, making it a good choice for scenarios where readability and conciseness are priorities.

Handling Arrays of Different Lengths

In real-world scenarios, you might need to zip arrays of different lengths. One approach is to fill the missing elements with undefined, ensuring the resulting array pairs always have two elements:

function zipArrays(array1, array2) {
    const maxLength = Math.max(array1.length, array2.length);
    let zippedArray = [];
    
    for (let i = 0; i < maxLength; i++) {
        zippedArray.push([array1[i] ?? undefined, array2[i] ?? undefined]);
    }
    
    return zippedArray;
}

This method uses the ?? operator to handle cases where an element is missing in one of the arrays by explicitly inserting undefined.

Using ES6 Features for a More Elegant Solution

You can leverage ES6 features like the spread operator (...) and Array.from() to create a more elegant zip function. This method is especially useful for zipping arrays of different lengths:

function zipArrays(array1, array2) {
    const maxLength = Math.max(array1.length, array2.length);
    return Array.from({ length: maxLength }, (_, i) => [array1[i], array2[i]]);
}

Here, Array.from() is used to create a new array of the desired length, and the callback function generates each pair by accessing the current index (i) in both arrays.

Conclusion

Zipping arrays is a common requirement in software development, and JavaScript provides several ways to implement this functionality, even in the absence of a built-in “zip” method. Depending on your specific needs—such as whether you expect arrays of equal length or if you need to handle arrays of different lengths gracefully—there’s a range of approaches from simple loops to more advanced functional programming techniques. Each method has its advantages, and the best choice depends on your particular scenario, priorities for readability, performance, and elegance.

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Anastasios Antoniadis

Anastasios Antoniadis is the founder and editor-in-chief of BORDERPOLAR... He is a software engineer, blogger, and avid gamer covering tech, gaming, and coding guides for over 4 years. He is a 2014 graduate of the Department of Informatics and Telecommunications of the University of Athens, an M.Sc. holder in Computer Science, and a Ph.D. student in Program Analysis.