codePointAt0

codePointAt0 is a concept used in programming libraries to obtain the Unicode code point at the start of a string, i.e., at position 0. It is commonly implemented as a wrapper or alias for a function that returns the first Unicode code point, taking into account how characters are encoded in the underlying string representation.

In languages that use UTF-16 encoding for strings, codePointAt0 may need to inspect two code units in order to resolve a single code point when the first character is outside the Basic Multilingual Plane. This means that a character such as a musical symbol or an emoji may be represented by a surrogate pair and treated as one code point rather than two separate units.

In JavaScript, the equivalent operation is String.prototype.codePointAt(0). It returns the numeric Unicode code point value at the requested position, or NaN if the position is out of range. Because code points above U+FFFF are encoded as surrogate pairs in UTF-16, codePointAt(0) will combine the high surrogate and low surrogate at index 0 if present, yielding a value from 0 to 0x10FFFF.

By contrast, charCodeAt(0) returns the UTF-16 code unit at index 0, which may be a single surrogate unit rather than a full code point when dealing with characters outside the BMP. This distinction is essential for correctly processing Unicode in environments that use UTF-16 encoding.

Examples: 'A'.codePointAt(0) yields 65; '𝄞'.codePointAt(0) yields 0x1D11E. Some APIs name the functionality differently or provide wrappers such as codePointAt0; understanding the difference between code units and code points is essential for correct Unicode handling.