String.prototype.toUpperCase()

The toUpperCase() method returns the calling string value converted to uppercase (the value will be converted to a string if it isn't one).

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

TypeError

When called on null or undefined, for example, String.prototype.toUpperCase.call(undefined).

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: function 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()