How to determine type of variable in typescript, array of numbers or array of strings?

Question

I have a variable like

const data = [];

In some places in my App, data looks like string array or number array

// data [1, 2, 3]
// data ["1", "2", "3"]

How can I get the type of this variable - type string array or number array ?

Is it possible to have both numbers and strings in an array ? — Romi Halasz, Dec 17 '19 at 07:39
It seems like you have *different* data in it at different points. Settle on just one - whatever makes sense for your use-case, then give the variable *that* type and then ensure you only have one type going into it. For example, if you declare it to be `number[]` then convert all input to numeric before adding into the array. — VLAZ, Dec 17 '19 at 07:39
@RomiHalasz Yes, no real problem with that `["hello", 42, true]` is valid. — VLAZ, Dec 17 '19 at 07:40
@0xD34F _Array of numbers or array of strings_ - `string[] | number[]` — Aleksey L., Dec 17 '19 at 08:16

Tomas Vancoillie · Answer 1 · 2019-12-17T07:47:32.667

2

Yes you can type your variables with a single type

// type data as an array of strings
const data: string[] = [];

// type data as an array of numbers
const data: number[] = [];

// type data as an array of objects
const data: object[] = [];

If you want to use a mix of types in your array, you can type it as any. But this may not be a good practise.

const data: any[] = [];

For more information about type in typescript, look here: Basic type documentation

For defining array with multiple types, look here: https://stackoverflow.com/a/29382420/1934484

edited Dec 17 '19 at 07:47

answered Dec 17 '19 at 07:39

Tomas Vancoillie

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This is what I need. But how to specify only lines or only numbers together? – Dec 17 '19 at 07:44
@0xD34F look here if you want to use multiple types https://stackoverflow.com/a/29382420/1934484 – Tomas Vancoillie Dec 17 '19 at 07:46

kevinSpaceyIsKeyserSöze · Answer 2 · 2019-12-17T09:11:06.653

2

You can define your array like this the typescript compiler will only allow strings

const array: Array<string> = [];

or if you want to have strings and numbers define it like this

const array: Array<string | number> = [];

You can also define it this way

const array: string[] = [];
const array: (string | number)[] = [];

edited Dec 17 '19 at 09:11

answered Dec 17 '19 at 08:01

kevinSpaceyIsKeyserSöze

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score 1 · Answer 3 · answered Dec 17 '19 at 07:42

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@Tomas Vancoillie has the right idea.

You can declare them with the : operator.

You can also infer the type with Array(), for example:

let myStringArray = Array<string>();

answered Dec 17 '19 at 07:42

Paul

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if specify Array Will it be (1) only strings and only numbers? Or (2)strings and numbers? – Dec 17 '19 at 07:47
@0xD34F strings and numbers. – VLAZ Dec 17 '19 at 07:50
Array is of Type T, which means , it will be strings and numbers. – Paul Dec 17 '19 at 07:50
@Paul How to set type T ? – Dec 17 '19 at 07:56
@0xD34F T means generic, it accepts everything. If you want to use anything, use the type – Paul Dec 17 '19 at 07:56
@0xD34F https://www.typescriptlang.org/docs/handbook/generics.html check this out. – Paul Dec 17 '19 at 07:57
[Related](https://stackoverflow.com/questions/931872/what-s-the-difference-between-array-and-while-declaring-a-javascript-ar) – kevinSpaceyIsKeyserSöze Dec 17 '19 at 08:18

score 0 · Answer 4 · answered Dec 17 '19 at 07:47

0

If you only need to get the type of the elements in the, since they are either string or number, you can just get the type of the first element, since all other elements have the same type:

typeof data[0] // string or number

answered Dec 17 '19 at 07:47

Romi Halasz

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How to determine type of variable in typescript, array of numbers or array of strings?

4 Answers4