Join in nested array in JavaScript

Joining elements from nested arrays in JavaScript requires flattening the array structure first. This article explores different approaches to join nested array elements with a semicolon separator.

The Problem

Consider this nested array:

const arr = ['zero', ['one', 'two', 'three', ['four', ['five', 'six', ['seven']]]]];
console.log("Original nested array:", arr);

Original nested array: [ 'zero', [ 'one', 'two', 'three', [ 'four', [Array] ] ] ]

We need to extract all elements and join them with semicolons to get:

zero;one;two;three;four;five;six;seven;

Using flat() Method (Modern Approach)

The flat(Infinity) method flattens arrays to any depth:

const arr = ['zero', ['one', 'two', 'three', ['four', ['five', 'six', ['seven']]]]];

const joinNestedArray = (nestedArr) => {
    return nestedArr.flat(Infinity).join(';') + ';';
};

console.log(joinNestedArray(arr));

zero;one;two;three;four;five;six;seven;

Using Recursive Approach

For environments without flat() support, use recursion:

const arr = ['zero', ['one', 'two', 'three', ['four', ['five', 'six', ['seven']]]]];

const buildStringRecursive = (arr = [], result = []) => {
    for (let i = 0; i < arr.length; i++) {
        if (Array.isArray(arr[i])) {
            buildStringRecursive(arr[i], result);
        } else {
            result.push(arr[i]);
        }
    }
    return result.join(';') + ';';
};

console.log(buildStringRecursive(arr));

zero;one;two;three;four;five;six;seven;

Using JSON.stringify() Approach

A creative approach using JSON parsing:

const arr = ['zero', ['one', 'two', 'three', ['four', ['five', 'six', ['seven']]]]];

const joinWithJSON = (nestedArr) => {
    return JSON.stringify(nestedArr)
        .replace(/[\[\]"]/g, '')
        .replace(/,/g, ';') + ';';
};

console.log(joinWithJSON(arr));

zero;one;two;three;four;five;six;seven;

Comparison

Conclusion

Use flat(Infinity) for modern environments due to its simplicity and performance. For older browsers, the recursive approach provides reliable cross-platform compatibility.