TypeScript: concat arrays with different element-types

Question

This is working.

let head = [["title", "value"], ["a", 1]];
let tail = [["b", 2], ["c", 3]];

let all = head.concat (tail);

The fine result is

[["title", "value"], ["a", 1], ["b", 2], ["c", 3]]

But what I need is this - and thats not working.

let head = [["title", "value"]];
let tail = [["a", 1], ["b", 2], ["c", 3]];

let all = head.concat (tail);

Error:

Argument of type '(string | number)[][]' is not assignable to parameter 
of type 'string[] | string[][]'. 
 Type '(string | number)[][]' is not assignable to type 'string[][]'. 
  Type '(string | number)[]' is not assignable to type 'string[]'. 
   Type 'string | number' is not assignable to type 'string'. 
    Type 'number' is not assignable to type 'string'.

It works if I make the numbers in tail to strings - what I can not do because of reasons.

So how can I make it work??

Thanks!

sunn0 · Accepted Answer · 2022-10-27 23:01:53Z

20

The recommended way to concat arrays these days is using es6 array spread. Type will be inferred correctly.

See MDN

const all = [...head, ...tail];

edited Oct 27, 2022 at 23:01

answered Sep 1, 2022 at 21:40

sunn0

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3 Comments

Rohit Gupta Over a year ago

Please explain your answer furtehr

Rohit Gupta Over a year ago

Surely you know how this works, it was flagged in review queues. :-)

Kev Over a year ago

Fyi, looking at some benchmarks, spread is slightly slower. Probably insignificant in 99% of cases but if you need high perf, take that into account.

Tsvetan Ganev · Accepted Answer · 2018-01-13 17:17:41Z

9

You can declare the type of head like this:

let head: [Array<string|number>] = [["title", "value"]];

This will remove the error and keep the type-safety in place.

answered Jan 13, 2018 at 17:17

Tsvetan Ganev

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1 Comment

pilchard Over a year ago

To narrow further let head: Array<[string, string|number]>

Pat Jones · Accepted Answer · 2022-02-02 09:27:36Z

3

You can just do this: [].concat(head, tail)

edited Feb 2, 2022 at 9:27

Pat Jones

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answered Jan 28, 2022 at 10:24

ThorEpNorway

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