String in JavaScript: working with text you cannot modify

Everything that comes from outside a program arrives as text. A form field, a URL parameter, a raw API response: the string is the default input format, and knowing how to slice one cleanly saves a great deal of time.

One property sets it apart from most other structures: it never changes. Every method returns a new string, which has immediate practical consequences.

Definition

A String is a sequence of characters, written between single quotes, double quotes or backticks. All three produce the same type, and only backticks allow variables to be interpolated.

const title = "  JavaScript for the web  ";

console.log(title.length);            // 26, spaces included
console.log(title.trim());            // "JavaScript for the web"
console.log(title.includes("web"));   // true
console.log(title.trim().toUpperCase());

const left = 3;
console.log(`There are ${left} seats left`);  // "There are 3 seats left"

Methods chain because each one returns a string, which lets you read a transformation from left to right in the order it happens.

The methods you reach for daily

console.log("a,b,c".split(","));        // ["a", "b", "c"]
console.log("Hello".slice(0, 3));       // "Hel"
console.log("price: 49".replace("49", "59"));  // "price: 59"
console.log("7".padStart(2, "0"));      // "07"

Immutability, and what it implies

A string never changes in place. Writing to a character by its index does nothing in sloppy mode, and throws in strict mode, which is what modules run in.

const word = "abc";
const other = word.replace("a", "z");

console.log(other);  // "zbc"
console.log(word);   // "abc", unchanged

The practical consequence is simple: the result of a method must always be captured. Forgetting the assignment is the most common string mistake, and it produces no message at all.

Frequently asked questions

Single and double quotes are strictly equivalent, the choice is a team convention. Backticks add two real capabilities: dropping a variable straight into the text, and writing a string across several lines. They are now the most used form whenever a variable has to be inserted.

Because the + operator concatenates as soon as one of the two values is a string, instead of adding. The number is turned into text automatically. This behavior belongs to Coercion (type conversion), and the fix is to convert explicitly with Number before any calculation.

Because length counts code units, not visible characters. Emoji and some symbols take two units, so "👍".length is 2. To count the characters actually displayed, the trick is to convert to an array with Spread (...), which splits correctly.