Operator '+' cannot be applied to types 'String' and 'String' in TypeScript

Question

I am new to TypeScript trying to play with it. But I face a wired problem. When I try to concatenate two String type using + operator it's give an error that is

Operator '+' cannot be applied to types 'String' and 'String'

My Code snap is

var firstName: String = 'Foo';
var lastName: String = 'Bar';
var name = firstName + lastName;

If I use string instead String or add extra ''it works fine. I checked, within JavaScript we can use + on two String object then why it's show error in TypeScript ? Is it bug or feature ? I definitely missing something. Detail explanation appreciated.

Answer 1

String is not the same as string. Always use string unless you really, really know what you're up to.

The upper-case name String refers to the boxed version of a primitive string. These should generally be avoided wherever possible -- they don't behave like normal strings in subtle and unexpected ways (for example typeof new String('hello') is "object", not "string").