String and Object types point to values stored in the "heap", similar to Java.
However, in TypeScript, string literals are typed as string. The type String refers to objects, while string refers to "primitive values". For more information, MDN has a detailed article published here.
The definition of Object is as follows (you may not understand its signature, but don't worry about it - it's not relevant):
interface Object {
constructor: Function;
toString(): string;
toLocaleString(): string;
valueOf(): Object;
hasOwnProperty(v: PropertyKey): boolean;
isPrototypeOf(v: Object): boolean;
propertyIsEnumerable(v: PropertyKey): boolean;
}
(Assuming you are familiar with Java's interface, I will skip explaining what an interface is)
In contrast, the type string has the following members:
- An index signature of number which is similar to the
charAt(int) method in Java,
Symbol.iterator, mostly identical to CharSequence's codePoint method in Java,- and various methods named after strings
TypeScript uses "structural typing", which differs from Java's "nominal typing" system. A key distinction between these systems is that structural typing assesses sub-typing based on the type's properties. In other words, in TypeScript, Hello and Hi are considered the same type, unlike in Java:
interface Hello {}
interface Hi {}
const hello: Hello = {};
// no compile errors
const hi: Hi = hello;
TypeScript evaluates the intersection set of properties to determine if a value can be assigned or passed. However, Object and string have no common properties at all! TypeScript raises an error to indicate this mismatch.
To resolve this, you can change String to string as suggested by other users.