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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 ?

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5
  • Is it possible to have both numbers and strings in an array ? Commented Dec 17, 2019 at 7:39
  • 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. Commented Dec 17, 2019 at 7:39
  • @RomiHalasz Yes, no real problem with that ["hello", 42, true] is valid. Commented Dec 17, 2019 at 7:40
  • @RomiHalasz No only number or only string. Commented Dec 17, 2019 at 7:42
  • @0xD34F Array of numbers or array of strings - string[] | number[] Commented Dec 17, 2019 at 8:16

4 Answers 4

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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

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2 Comments

This is what I need. But how to specify only lines or only numbers together?
@0xD34F look here if you want to use multiple types stackoverflow.com/a/29382420/1934484
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)[] = [];
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1

@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>();
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7 Comments

if specify Array<string | number> Will it be (1) only strings and only numbers? Or (2)strings and numbers?
@0xD34F strings and numbers.
Array is of Type T, which means , it will be strings and numbers.
@Paul How to set type T ?
@0xD34F T means generic, it accepts everything. If you want to use anything, use the type <any>
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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

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