What is the difference between String() and new String() in Typescript? What problems we might run into if mixing those or using one vs the other?
I know that if we compare two strings then they will be equal like:
String('hello') === 'hello' // returns true
String literals (denoted by double or single quotes) and strings returned from String calls in a non-constructor context (that is, called without using the new keyword) are primitive strings. JavaScript automatically converts primitives to String objects, so that it's possible to use String object methods for primitive strings. In contexts where a method is to be invoked on a primitive string or a property lookup occurs, JavaScript will automatically wrap the string primitive and call the method or perform the property lookup.
let s_prim = 'foo'
let s_obj = new String(s_prim)
console.log(typeof s_prim) // Logs "string"
console.log(typeof s_obj) // Logs "object"
String primitives and String objects also give different results when using eval(). Primitives passed to eval are treated as source code; String objects are treated as all other objects are, by returning the object. For example:
let s1 = '2 + 2' // creates a string primitive
let s2 = new String('2 + 2') // creates a String object
console.log(eval(s1)) // returns the number 4
console.log(eval(s2)) // returns the string "2 + 2"
For these reasons, the code may break when it encounters String objects when it expects a primitive string instead, although generally, authors need not worry about the distinction.
A String object can always be converted to its primitive counterpart with the valueOf() method.
console.log(eval(s2.valueOf())) // returns the number 4
