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Learn Data Structures and Algorithms in JavaScript

String Immutability

Explore the concept of string immutability in JavaScript and understand its impact on modifying strings, designing algorithms, and optimizing performance. This lesson clarifies why strings cannot be changed in place and highlights how new strings are created to reflect modifications.

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By this point, you know that individual characters in strings can be accessed and traversed, and that the length of a string can be determined using the same ideas that apply to arrays. That structural similarity makes strings feel familiar. However, there is one place where strings and arrays part ways in a significant and consequential way, and it shows up the moment you try to modify a string. Understanding this difference is not just a matter of knowing a language rule. It directly affects how string algorithms are designed, how their performance is analyzed, and why certain patterns appear in string code again and again.

Attempting to modify a string

Consider a simple array of integers. Changing the first element is a straightforward operation in which the value at that position is simply overwritten.

Javascript (babel-node)
let arr = [1, 2, 3, 4, 5];
arr[0] = 99;
console.log(arr);  // [99, 2, 3, 4, 5]

Now consider trying to do the same thing with a string.

Javascript (babel-node)
let s = "hello";
s[0] = 'H';
console.log(s);  // "hello"

The result shows that a character in a string cannot be replaced in the same way that an element in an array can be replaced. Unlike Python, JavaScript does not raise an error here. The attempted modification is simply ignored because strings are immutable.

This difference is not accidental. It reflects a fundamental distinction between the two data types. An array allows direct modification of its contents, whereas a string does not. ...