JS: String Constructor

String()

Return a empty string, same as "".

String() === ""

String(value)

Convert value to string Primitive Value , return it.

// list of String(primitiveValue) for all possible primitiveValue

// undefined type
console.log(String(undefined) === "undefined");

// null type
console.log(String(null) === "null");

// boolean primitives
console.log(String(true) === "true");
console.log(String(false) === "false");

// number primitives
console.log(String(0) === "0");
console.log(String(+0) === "0");
console.log(String(-0) === "0");
console.log(String(12) === "12");
console.log(String(12.3) === "12.3");

// number primitives NaN and Infinity
console.log(String(NaN) === "NaN");
console.log(String(Infinity) === "Infinity");

// symbol primitives
console.log(String(Symbol()) === "Symbol()");
console.log(String(Symbol("xyz")) === "Symbol(xyz)");

if argument is Object Type , the result is the value of calling the function obj[Symbol.toPrimitive] if this method exist, or obj.toString(), or obj.valueOf(), in that order.

// convert array to string
const aa = [3, 4, 5];

console.log(String(aa) === "3,4,5");

// String(myArray) is the same as myArray.toString()
console.log(String(aa) === aa.toString());

// convert object to string
const jj = { "p": 4 };

console.log(String(jj) === "[object Object]");

// String(myObj) is the same as myObj.toString()
console.log(String(jj) === jj.toString());

new String(value)

Like String(value), but returns a Array-Like Object that represents the string. The property values are String Code Units of the string.

const xx = "abc";
const yy = new String(xx);
console.log(typeof yy === "object");
console.log(yy.length === 3);
console.log(yy["0"] === "a");
console.log(yy["1"] === "b");
console.log(yy["2"] === "c");

〔see Difference Between String() vs new String()〕