Joining two Array<['string', 'string']> throws compiler warning

Question

I'm trying to join two Array like this:

const param = null
const optionsHeaders: Array<['string', 'string']> = param || []
const sesHeaders = [
    ['X-SES-CONFIGURATION-SET', 'config-set']
]
const headers: Array<['string', 'string']> = [...optionsHeaders, ...sesHeaders]
console.log(headers)

The result is as expected:

[
  [
    "X-SES-CONFIGURATION-SET",
    "config-set"
  ]
]

But the TypeScript compiler is complaining:

Type 'string[][]' is not assignable to type '["string", "string"][]'.
  Type 'string[]' is missing the following properties from type '["string", "string"]': 0, 1

const headers: Array<['string', 'string']> = [...optionsHeaders, ...sesHeaders]

What am I missing here?

: string = all possible strings, : 'string' = only the string 'string'. Yes, values can be types! — Nino Filiu
– Nino Filiu, Commented Jul 11, 2019 at 17:14

dezfowler · Accepted Answer · 2019-07-11 19:29:48Z

3

You need the type on sesHeaders as, by default an array literal [ 'foo', 'bar'] will have an inferred type of string[] and not [string, string].

Also, your 'string' don't need the single quotes because, as @nino-filio points out, constant values can be types too so by saying 'string' you're saying "only a string of value 'string'".

type Headers = [ string, string ][];

const param = null
const optionsHeaders: Headers = param || []
const sesHeaders: Headers = [
    ['X-SES-CONFIGURATION-SET', 'config-set']
]
const headers: Headers = [...optionsHeaders, ...sesHeaders]
console.log(headers);

edited Jul 11, 2019 at 19:29

answered Jul 11, 2019 at 17:06

dezfowler

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