Difference between type[] and [type] in typescript

Question

Lets say we have two interfaces:

interface WithStringArray1 {     property: [string] }  interface WithStringArray2 {     property: string[] } 

Lets declare some variables of these types:

let type1:WithStringArray1 = {    property: [] }  let type2:WithStringArray2 = {     property: [] } 

The first initialisation fails with:

TS2322: Type '{ property: undefined[]; }' is not assignable to type 'WithStringArray1'. Types of property 'property' are incompatible. Type 'undefined[]' is not assignable to type '[string]'. Property '0' is missing in type 'undefined[]'. 

The second one is ok.

What is the difference between [string] and string[]?

Answer 1

• [string] denotes a Tuple of type string
• string[] denotes an array of strings

The correct usage of the Tuple in your case would be:

let type2:WithStringArray2 = {     property: ['someString'] }; 

See Documentation