StringprototypecodePointAt0

String.prototype.codePointAt(0) refers to the JavaScript String method that returns the numeric Unicode code point at the first position of a string. It is similar in purpose to charCodeAt but differs in how it handles characters that are represented by surrogate pairs in UTF-16. When used with 0, the method will return the full code point for a character that starts at the first position if that character is encoded as a surrogate pair.

The argument pos is converted to an integer. If pos is negative or greater than or equal to the string’s length, the result is undefined. If the unit at position 0 is a high surrogate and the following unit is a low surrogate, codePointAt(0) returns the combined code point value. Otherwise, it returns the numeric value of the code unit at position 0.

Examples help illustrate the behavior. For a simple ASCII string like "A", "A".codePointAt(0) yields 65. For a character outside the Basic Multilingual Plane, such as the emoji "😀" (U+1F600), "😀".codePointAt(0) yields 0x1F600 (128512). In a mixed string like "A😀" the call "A😀".codePointAt(1) returns 0x1F600, since position 1 starts the surrogate pair that forms the single code point.

Edge cases include strings starting with a lone high surrogate (not followed by a low surrogate), where codePointAt(0) returns the high surrogate’s code unit value. If the string is empty, or pos is out of range, the result is undefined.

See also: String.prototype.charCodeAt, Unicode code points, surrogate pairs.