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

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[]?

Jo Liss · Accepted Answer · 2024-03-22 17:42:35Z

40

[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

edited Mar 22, 2024 at 17:42

Jo Liss

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answered Apr 20, 2016 at 11:18

haim770

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Comments

Derviş Kayımbaşıoğlu · Accepted Answer · 2018-12-10 14:18:52Z

4

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.

answered Dec 10, 2018 at 14:18

Derviş Kayımbaşıoğlu

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