String(123) returns the string '123'. The String() function is used for explicit type conversion to string in JavaScript. It can convert any value to a string representation. For numbers, it simply returns the number as a string. For booleans, String(true) returns 'true' and String(false) returns 'false'. For null, it returns 'null', and for undefined, it returns 'undefined'. Objects are typically converted to strings like '[object Object]' unless they have a custom toString() method.