How to filter a dictionary in Typescript?

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Do you want to filter a dictionary in Typescript? In the Typescript tutorial, I have explained how to filter a dictionary in Typescript.

Filter a dictionary in Typescript

Filtering a dictionary in TypeScript involves selecting specific key-value pairs that meet certain criteria from an existing dictionary. I will show you 3 different methods to filter a dictionary in Typescript.

  • Using Object.keys() and Array.prototype.filter()
  • Using Object.entries() and reduce()
  • Using Custom Filter Function

1. Using Object.keys() and Array.prototype.filter()

You can filter a dictionary in Typescript using Object.keys() and Array.prototype.filter(). Here is a complete code:

function filterDictionaryByKey(dict: { [key: string]: any }, predicate: (key: string) => boolean): { [key: string]: any } {
  let result: { [key: string]: any } = {};
  Object.keys(dict).filter(key => predicate(key)).forEach(filteredKey => {
    result[filteredKey] = dict[filteredKey];
  });
  return result;
}

// Example usage:
const myDictionary = {
  apple: 1,
  banana: 2,
  cherry: 3,
  date: 4
};

const filteredDictionary = filterDictionaryByKey(myDictionary, key => key.startsWith('a'));
console.log(filteredDictionary);

You can see the output below in the screenshot after running the code using Visual Studio Code.

filter dictionary in Typescript

2. Using Object.entries() and reduce()

Another method is using Object.entries() in combination with reduce() to filter the dictionary in Typescript.

function filterDictionaryByValue(dict: { [key: string]: number }, predicate: (value: number) => boolean): { [key: string]: number } {
  return Object.entries(dict).reduce((acc, [key, value]) => {
    if (predicate(value)) {
      acc[key] = value;
    }
    return acc;
  }, {} as { [key: string]: number });
}

// Example usage:
const filteredByValue = filterDictionaryByValue(myDictionary, value => value >= 3);
console.log(filteredByValue);

Once you run the code using Visual Studio Code, you can see the output below:

{ cherry: 3, date: 4 }
typescript filter dictionary

3. Using Custom Filter Function

You can also create a function that iterates over the dictionary entries to filter a dictionary in Typescript.

function customFilter<T>(dict: { [key: string]: T }, filterFn: (key: string, value: T) => boolean): { [key: string]: T } {
  let result: { [key: string]: T } = {};
  for (let [key, value] of Object.entries(dict)) {
    if (filterFn(key, value)) {
      result[key] = value;
    }
  }
  return result;
}

// Example usage:
const customFilteredDictionary = customFilter(myDictionary, (key, value) => value % 2 === 0);
console.log(customFilteredDictionary);

Conclusion

In this Typescript tutorial, I have explained how to filter a dictionary in Typescript.

  • Using Object.keys() and Array.prototype.filter()
  • Using Object.entries() and reduce()
  • Using Custom Filter Function

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