String Reversal Algorithm

Reverse a string. "hello" becomes "olleh". It's the "Hello, World!" of algorithm problems — but there's more depth here than you'd expect.

Three approaches, each with different trade-offs.

Iterative

Walk backwards through the string. Build a new one character by character.

function reverseIterative(str) {
    let reversed = '';
    for (let i = str.length - 1; i >= 0; i--) {
        reversed += str[i];
    }
    return reversed;
}

Time: O(n). Space: O(n) for the new string. In JavaScript, string concatenation in a loop can be O(n²) in older engines because strings are immutable — each += creates a new string. Modern engines optimize this, but it's worth knowing.

Recursive

Take the first character, put it at the end. Recurse on the rest.

function reverseRecursive(str) {
    if (str.length <= 1) return str;
    return reverseRecursive(str.slice(1)) + str[0];
}

Time: O(n). Space: O(n) call stack. This will blow the stack on very long strings. Elegant but impractical for large inputs.

Built-in

Split into an array, reverse, join back. One line.

function reverseBuiltin(str) {
    return str.split('').reverse().join('');
}

Time: O(n). Space: O(n). This is what I'd write in production. It's clear, fast enough, and everyone on the team understands it instantly.

Which one to use?

In an interview, they usually want the iterative or two-pointer approach to prove you understand the mechanics. In production, use the built-in. Readability wins when performance is equivalent.

One gotcha: split('') breaks on multi-byte Unicode characters (emojis, etc.). Use [...str] spread instead if that matters.