None.

A new string representing the calling string converted to upper case.

The toUpperCase() method returns the value of the string converted to uppercase. This method does not affect the value of the string itself since JavaScript strings are immutable.

This method will convert any non-string value to a string, when you set its this to a value that is not a string:

const a = String.prototype.toUpperCase.call({
  toString() {
    return "abcdef";
  },
});

const b = String.prototype.toUpperCase.call(true);

// prints out 'ABCDEF TRUE'.
console.log(a, b);

• String.prototype.toLocaleLowerCase()
• String.prototype.toLocaleUpperCase()
• String.prototype.toLowerCase()