CODEWITHSUNDEEP
typescript
Ludevik
Ludevik

Reputation: 7264

Difference between type[] (array) and [type] (tuple)

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[]?

Upvotes: 36

Views: 7045

Answers (2)

haim770
haim770

Reputation: 49105

  • [string] denotes a tuple with exactly one string element. Tuples are ordered lists of items, finite and usually fixed sized.
  • string[] denotes an array of strings. Arrays will have variable size, including even a size of zero.

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

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

See Documentation

Upvotes: 40

Derviş Kayımbaşıoğlu
Derviş Kayımbaşıoğlu

Reputation: 30625

if we look tuple with three variable. you can see the difference clearly.

let t: [number, string?, boolean?];
t = [42, "hello", true];

let tuple : [string] is tuple(string) whereas let arr : string[]is array of string.

Upvotes: 4

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