How use array as const to specify possible indexes in index signature TypeScript?

Question

I want to make an array, which will specify, which strings will index properties inside object and that object properties only contain functions.

What I do understand, is that I can create union type from just array:

My working case is a reducer action (instruction) in React reducer (but it can be used elsewhere):

const InstructionsArray = [
    'HANDLE_BUTTON_CONVERT_INPUT',
    'HANDLE_CHECKBOX_CONVERT_ON_INPUT',
    'HANDLE_CHECKBOX_DISPLAY_TAGS',
    'HANDLE_TEXTAREA_INPUT',
    'HANDLE_RUNTIME'
] as const;

Then I have type object with property:

instruction: typeof InstructionsArray[number]

Of course, normal array would allow us to use any string as indexes, but since we casted it as const, it now only allows for the strings, we specified in array.

What I actually want, is to have an object, which will have some property names as string and all properties will have value of function. I would like to specify only those property names (string), but as every value will be a function, I would not like to specify it more than once, so it can be taken as granted.

Current code looks like that:

const changersLabels = [
    'inventory',
    'quest'
] as const;

But the 'same' way does not work on object like that:

keyboardRouter(changers: {[key:typeof changersLabels[number]:Function]}) {

    //some stuff

}

What am I doing wrong and is that even possible?

jcalz · Accepted Answer · 2020-11-04 21:34:15Z

5

Looks like you just want a mapped type where the keys are typeof changersLabels[number] and the properties are all Function:

function keyboardRouter(changers: { [K in typeof changersLabels[number]]: Function }) {
  changers.inventory // Function
  changers.quest // Function
}

A mapped type looks a little like a type with an index signature, but that only works where the index is of type string or of type number. For other types like a union of string literals, you need a mapped type with the in operator.

Which could be slightly refactored to give a name to your keys and to use the built-in Record utility type:

type ChangersLabels = typeof changersLabels[number];
function keyboardRouter2(changers: Record<ChangersLabels, Function>) {
  changers.inventory // Function
  changers.quest // Function
}

Playground link to code

answered Nov 4, 2020 at 21:34

jcalz

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