Trouble refactoring Typescript array into JSON object?

Question

I have a function that breaks a string into an array of keyword substrings where "hello" gives

keywords
   0: "h"
   1: "he"
   2: "hel"
   3: "hell"
   5: "hello"

I need to refactor into a json object:

keywords
   "h": true
   "he": true
   "hel": true
   "hell": true
   "hello": true

The working array function is

function createKeywords (name: string) {
  const keywords: string[] = [];
  let keyword = "";
  name.split("").forEach((letter) => {
    keyword += letter;
    keywords.push(keyword);
  });
  return keywords;
};

and my broken refactor attempt is

function createKeywords (name: string) {
  const keywords: {text: string, value: boolean}[] = [];
  let keyword = "";
  name.split("").forEach((letter) => {
    keyword += letter;
    keywords[keyword] = true; // Element implicitly has an 'any' type because index expression is not of type 'number'.
  });
  return keywords;
};

Which gives the above error?

There is no such thing as a "json object" or "json array". JSON is a text representation of some data, usually an object or an array. It is just plain text and nothing more. It can be used to restore data structures similar to those used to create it. There isn't any trace of JSON in this question. What you want is to create a JavaScript object. — axiac
– axiac, Commented Nov 7, 2020 at 15:17

Aplet123 · Accepted Answer · 2020-11-07 14:54:53Z

1

There are a couple changes that need to be made here. First of all, keywords should be an object, not an array of objects. Second, {text: string, value: boolean} is an object that has a property text that is a string, and a property value that is a boolean. What you're looking for is an object with string keys and boolean values, or {[key: string]: boolean}. Here's the updated code:

function createKeywords (name: string): {[key: string]: boolean} {
  const keywords: {[key: string]: boolean} = {};
  let keyword = "";
  name.split("").forEach((letter) => {
    keyword += letter;
    keywords[keyword] = true;
  });
  return keywords;
}

Alternatively, {[key: string]: boolean} can be expressed as Record<string, boolean>.

answered Nov 7, 2020 at 14:54

Aplet123

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Comments

marcopiii · Accepted Answer · 2020-11-07 15:06:10Z

1

This is due to the fact that you are using an array with non-numeric index to store key:value pairs. Indeed, if suppressing the error, the result is

[ h: true, he: true, hel: true, hell: true, hello: true ]

You should use an object. This is a way you can achieve it

function createKeywords(name: string) {
  let out: { [k: string]: boolean } = {};
  name.split("").forEach((_, i) => {
    const key = name.substr(0, i + 1);
    out[key] = true;
  });
  return out;
}

which returns

{ h: true, he: true, hel: true, hell: true, hello: true }

edited Nov 7, 2020 at 15:06

answered Nov 7, 2020 at 15:00

marcopiii

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Comments

axiac · Accepted Answer · 2020-11-07 15:27:06Z

1

You can modify the function createKeywords() to return an object instead of an array or you can let it unchanged and use the value it returns to produce the expected result.

For example:

function createKeywords (name) {
  const keywords = [];
  let keyword = "";
  name.split("").forEach((letter) => {
    keyword += letter;
    keywords.push(keyword);
  });
  return keywords;
};

const keywords = createKeywords('hello');
const asObject = Object.fromEntries(
  keywords.map((k) => [k, true])
);

console.log(keywords);
console.log(asObject);

Read about Object.fromEntries() and Array.map().

answered Nov 7, 2020 at 15:27

axiac

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