The result of String(Symbol('foo')) is the string 'Symbol(foo)'. Symbols are a primitive data type in JavaScript introduced in ES6, and they can be converted to strings explicitly using the String() function or the toString() method. However, it's important to note that symbols cannot be implicitly converted to strings, so an expression like Symbol('foo') + '' would throw a TypeError. This is one of the ways Symbols differ from other primitive types in JavaScript and helps maintain their uniqueness.