JS: String.fromCharCode

String.fromCharCode ( …codeUnits )

Return a string, where each character in string has corresponding codeUnits.

[see JS: Character, Code Unit, Code Point]

Each of codeUnit should be a integer from 0 to 65535 inclusive. (Note: 65535 = 2^16 -1). If it is greater than 65535, a value of modulo(codeUnit, 2^16) is used.

(it's possible to use String.fromCharCode() to create a string that contains a character with Unicode codepoint greater than 65535. You have to find the pair of 16 bits values (called surrogate pairs) to form 1 such character.)

Note: most of the time, you probably want String.fromCodePoint() instead.

[see JS: String.fromCodePoint]

// convert sequence of unicode codepoint to string

console.log (
    String.fromCharCode ( 97, 98) === "ab"
); // true

// 97 is the codepoint for letter “a”
// 98 is the codepoint for letter “b”

// String.fromCharCode won't work as expected if codepoint is greater than 65535 (that's 2^16 )

const tt = String.fromCharCode ( 128514 );

// 😂
// name: FACE WITH TEARS OF JOY
// codepoint decimal: 128514
// codepoint hexadecimal: 1f602

console.log ( tt ); // prints , WRONG

Reference

ECMAScript 2015 §Text Processing#sec-string.fromcharcode