What is the best way to include a string index signature in a preexisting Array?

Currently, I am immersed in styled-system and attempting to define a pattern that is frequently employed within the library.

const space: { [key: string]: string } = [
  '0.25rem',
  '0.5rem',
  '1rem',
  '2rem',
  '4rem',
  '6rem',
];

space.xs = space[0];
space.sm = space[1];
space.md = space[2];
space.lg = space[3];
space.xl = space[4];
space.xxl = space[5];

An issue arises with the following error:

Type 'string[]' is not assignable to type '{ [key: string]: string; }'.
  Index signature is missing in type 'string[]'.ts(2322)

I am uncertain about how to combine a number key and a string key for an Array index or how to merge the object and array. Although it functions as intended in vanilla javascript. The quick solution is to assign string to any, but my curiosity lies in understanding why this doesn't behave as anticipated. I would assume that an associative array pattern should be permissible.

typescript

Answer №1

Unique Answer

If you're looking to incorporate types into your current JavaScript code, consider following this approach.

Take a look at how it's done by experimenting in the online playground.

type Props = Record<"xs" | "sm" | "md" | "lg" | "xl" | "xxl", string>;
type ArrayWithProps = string[] & Partial<Props>;

const space: ArrayWithProps = [
  "0.25rem",
  "0.5rem",
  "1rem",
  "2rem",
  "4rem",
  "6rem"
];

space.xs = space[0];
space.sm = space[1];
space.md = space[2];
space.lg = space[3];
space.xl = space[4];
space.xxl = space[5];

Dive Deeper

The process begins by defining Props utilizing the Record type feature. This built-in type specifies an index signature defining each type precisely.

type Props = {
  [K in "xs" | "sm" | "md" | "lg" | "xl" | "xxl"]: string;
};

This defined index signature works effectively compared to using [K in string], as we opt for specific string literal types over a broader definition of string.

Subsequently, a new type named ArrayWithProps is crafted. It merges the characteristics of both string[] and Props while making each property optional with the aid of Partial.

The end result successfully clears all type checks without altering the original JavaScript code in any way.

Answer №2

There is a mistake in the object notation being used.

{ [key: string]: string } implies an object where both keys and values are strings. However, in this case, what you actually have is an array of strings.

To rectify this, the correct notation should be as follows:

const space: string[] = [
  '0.25rem',
  '0.5rem',
  '1rem',
  '2rem',
  '4rem',
  '6rem',
];

If you still prefer to use an object structure, it should look like this:

const space: { [key: string]: string } = {
  xs: '0.25rem',
  sm: '0.5rem',
  md: '1rem',
  lg: '2rem',
  xl: '4rem',
  xxl: '6rem',
};

Answer №3

TypeScript incorporates static typings and type checking during compilation. Consequently, trying to assign a string[] to { [key: string]: string } results in an error. If you wish to utilize an object instead, you can achieve it like this:

const space: { [key: string]: string } = {};

for (const s of ['0.25rem', '0.5rem', '1rem', '2rem', '4rem', '6rem']) {
  space[s] = s;
}

console.log(space);

// { 0.25rem: "0.25rem", 0.5rem: "0.5rem", 1rem: "1rem", 2rem: "2rem", 4rem: "4rem", 6rem: "6rem" }

Edit:

You have the option to use a Map:

const map: Map<string, string> = new Map([
  ['xs', '0.25rem'],
  ['sm', '0.5rem'],
  ['md', '1rem'],
  ['lg', '2rem'],
  ['xl', '4rem'],
  ['xxl', '6rem']
]);

console.log(map.get('xs'));
// 0.25rem

console.log(Array.from(map.values()));
// ["0.25rem", "0.5rem", "1rem", "2rem", "4rem", "6rem"]

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