String() constructor

thing

Anything to be converted to a string.

When String is called as a constructor (with new), it creates a String object, which is not a primitive.

When String is called as a function, it coerces the parameter to a string primitive. Symbol values would be converted to "Symbol(description)", where description is the description of the Symbol, instead of throwing.

String function and String constructor produce different results:

const a = new String("Hello world"); // a === "Hello world" is false
const b = String("Hello world"); // b === "Hello world" is true
a instanceof String; // is true
b instanceof String; // is false
typeof a; // "object"
typeof b; // "string"

Here, the function produces a string (the primitive type) as promised. However, the constructor produces an instance of the type String (an object wrapper) and that's why you rarely want to use the String constructor at all.

String() is the only case where a symbol can be converted to a string without throwing, because it's very explicit.

const sym = Symbol("example");
`${sym}`; // TypeError: Cannot convert a Symbol value to a string
"" + sym; // TypeError: Cannot convert a Symbol value to a string
"".concat(sym); // TypeError: Cannot convert a Symbol value to a string

const sym = Symbol("example");
String(sym); // "Symbol(example)"

• Text formatting in the JavaScript Guide