string vs String

Written byPhuoc Nguyen

#Difference

`string` refers to the JavaScript's primitive types and can be created by using either literals (single or double quotes) or `String` function (without the `new` keyword).

The three declarations below create the same string:

const message = 'hello';
const message = 'hello';
const message = String('hello');

We often use `typeof variable === 'string'` to check if a given variable is a primitive string.

`String` on the other hand is an object that wraps the primitive string, and used to manipulate strings. We can create an instance of `String` from the constructor such as `new String(...)`:

const message = new String('hello');

In order to check whether a variable is an instance of `String` object, we have to use the `instanceof` operator:

if (variable instanceof String) {
    ...
}

#Good to know

Given the declarations at the top of this page, you can assign a `String` object to a primitive `string` variable:

let foo: String = 'foo';
let bar: string = 'bar';

foo = bar; // OK

As the time of writing, `String` is declared as an interface so that the `string` is treated as a subtype of `String`. Assigning `foo = bar` therefore does not cause any problem.

But doing the opposite assignment will throw an error:

// ERROR: Type 'String' is not assignable to type 'string'.
// 'string' is a primitive, but 'String' is a wrapper object.
bar = foo;

#Good practice

According to the official TypeScript's Do's and Don'ts, it is recommended to not use `Number`, `String`, `Boolean`, `Symbol`, or `Object`.

// Do NOT
const reverse = (s: String): String => {
    ...
}

// Do
const reverse = (s: string): string => {
    ...
}

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