String.prototype.charAt()

index

Zero-based index of the character to be returned. Converted to an integer — undefined is converted to 0.

A string representing the character (exactly one UTF-16 code unit) at the specified index. If index is out of the range of 0 – str.length - 1, charAt() returns an empty string.

Characters in a string are indexed from left to right. The index of the first character is 0, and the index of the last character in a string called str is str.length - 1.

Unicode code points range from 0 to 1114111 (0x10FFFF). charAt() always returns a character whose value is less than 65536, because the higher code points are represented by a pair of 16-bit surrogate pseudo-characters. Therefore, in order to get a full character with value greater than 65535, it is necessary to retrieve not only charAt(i), but also charAt(i + 1) (as if manipulating a string with two characters), or to use codePointAt(i) and String.fromCodePoint() instead. For information on Unicode, see UTF-16 characters, Unicode code points, and grapheme clusters.

charAt() is very similar to using bracket notation to access a character at the specified index. The main differences are:

• charAt() attempts to convert index to an integer, while bracket notation does not, and directly uses index as a property name.
• charAt() returns an empty string if index is out of range, while bracket notation returns undefined.

The following example displays characters at different locations in the string "Brave new world":

const anyString = "Brave new world";
console.log(`The character at index 0   is '${anyString.charAt()}'`);
// No index was provided, used 0 as default

console.log(`The character at index 0   is '${anyString.charAt(0)}'`);
console.log(`The character at index 1   is '${anyString.charAt(1)}'`);
console.log(`The character at index 2   is '${anyString.charAt(2)}'`);
console.log(`The character at index 3   is '${anyString.charAt(3)}'`);
console.log(`The character at index 4   is '${anyString.charAt(4)}'`);
console.log(`The character at index 999 is '${anyString.charAt(999)}'`);

These lines display the following:

The character at index 0   is 'B'

The character at index 0   is 'B'
The character at index 1   is 'r'
The character at index 2   is 'a'
The character at index 3   is 'v'
The character at index 4   is 'e'
The character at index 999 is ''

charAt() may return lone surrogates, which are not valid Unicode characters.

const str = "𠮷𠮾";
console.log(str.charAt(0)); // "\ud842", which is not a valid Unicode character
console.log(str.charAt(1)); // "\udfb7", which is not a valid Unicode character

To get the full Unicode code point at the given index, use an indexing method that splits by Unicode code points, such as String.prototype.codePointAt() and spreading strings into an array of Unicode code points.

const str = "𠮷𠮾";
console.log(String.fromCodePoint(str.codePointAt(0))); // "𠮷"
console.log([...str][0]); // "𠮷"

• String.prototype.indexOf()
• String.prototype.lastIndexOf()
• String.prototype.charCodeAt()
• String.prototype.codePointAt()
• String.prototype.split()
• String.fromCodePoint()
• JavaScript has a Unicode problem by Mathias Bynens (2013)