string vs String
Written byPhuoc Nguyen
Created
25 Aug, 2020
Last updated
20 Apr, 2022
Category
TypeScript
Contributors
`string` and `String` are valid TypeScript types. The following declarations are valid:js
let foo: String = 'foo';
let bar: string = 'bar';#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:
js
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(...)`:js
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:js
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:js
let foo: String = 'foo';
let bar: string = 'bar';
foo = bar; // OKAs 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:
js
// 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`.js
// Do NOT
const reverse = (s: String): String => {
...
}
// Do
const reverse = (s: string): string => {
...
}
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Phước Nguyễn