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What Is a String?

In programming, a string is a sequence of characters used to represent text. Strings are one of the most fundamental data types across all programming languages. The word "string" comes from the metaphor of threading characters together like beads on a string.

How a string is stored, indexed, and measured depends critically on the programming language — specifically on how that language represents characters internally.

Strings and Unicode

Before Unicode, strings were simple: each character was one byte, the encoding was fixed (ASCII, Latin-1, etc.), and string length equaled byte count. Unicode broke this assumption. A Unicode string contains characters from any script, and the same abstract string can be encoded as different byte sequences depending on the encoding (UTF-8, UTF-16, UTF-32).

Modern languages differ in their internal string representation:

Python String Fundamentals

# Python 3 str = Unicode string (sequence of code points)
s = "Hello, 世界 🌍"

len(s)          # 11 code points
s[7]            # "界" — indexed by code point
s[-1]           # "🌍" — single emoji code point

# bytes vs str
b = s.encode("utf-8")   # bytes object
len(b)                  # 19 (UTF-8 bytes)
b.decode("utf-8") == s  # True

# Byte length varies by encoding
s.encode("utf-8")   # variable: ASCII=1, CJK=3, emoji=4 bytes
s.encode("utf-16")  # 2 bytes per BMP char, 4 for supplementary
s.encode("utf-32")  # always 4 bytes per code point

# String methods work on code points
"café".upper()          # "CAFÉ"
"résumé".casefold()     # "résumé"

JavaScript String Quirks

// JS strings are UTF-16 — supplementary chars have length 2
const simple = "Hello";
simple.length;   // 5

const emoji = "🌍";
emoji.length;    // 2 (two UTF-16 code units)
emoji[0];        // "\uD83C" (high surrogate — not meaningful alone)

// Code points (correct count):
[...emoji].length;           // 1
emoji.codePointAt(0);        // 127757 (0x1F30D)

// Iterating by code point (ES6+)
for (const char of "😀abc") {
  console.log(char);  // "😀", "a", "b", "c"
}

Strings as Immutable Sequences

In Python, Java, and JavaScript, strings are immutable: you cannot change a character in place. All "modification" operations create new string objects.

s = "hello"
s[0] = "H"       # TypeError: 'str' object does not support item assignment
s = "H" + s[1:]  # Creates new string "Hello"

String Interning

Many languages intern (cache and reuse) string objects for short or frequently used strings. In Python, string literals and identifiers are typically interned; in Java, string literals in the string pool are interned. This means two variables holding the same short string value may reference the same object in memory.

a = "hello"
b = "hello"
a is b  # True (interned)

c = "".join(["h","e","l","l","o"])
c is a  # May be False (dynamically created)

Quick Facts