How can I instantiate a dictionary in JavaScript where all keys map to the same value?

In JavaScript, you can create a dictionary (object) where all keys map to the same value using several approaches. This is useful when you need to initialize multiple properties with identical values.

Method 1: Using a for...of Loop

The most straightforward approach is to iterate through an array of keys and assign the same value to each:

const keys = ['Name1', 'Name2', 'Name3'];
const dictionary = {};

for (const key of keys) {
    dictionary[key] = 'John';
}

console.log(dictionary);

{ Name1: 'John', Name2: 'John', Name3: 'John' }

Method 2: Using Object.fromEntries() with map()

A more functional approach using Object.fromEntries() combined with map():

const keys = ['Name1', 'Name2', 'Name3'];
const defaultValue = 'John';

const dictionary = Object.fromEntries(
    keys.map(key => [key, defaultValue])
);

console.log(dictionary);

{ Name1: 'John', Name2: 'John', Name3: 'John' }

Method 3: Using reduce()

You can also use the reduce() method to build the dictionary:

const keys = ['Name1', 'Name2', 'Name3'];
const defaultValue = 'John';

const dictionary = keys.reduce((acc, key) => {
    acc[key] = defaultValue;
    return acc;
}, {});

console.log(dictionary);

{ Name1: 'John', Name2: 'John', Name3: 'John' }

Method 4: Using a Helper Function

For reusability, create a helper function:

function createDictionary(keys, value) {
    const dictionary = {};
    for (const key of keys) {
        dictionary[key] = value;
    }
    return dictionary;
}

// Usage
const userNames = createDictionary(['admin', 'user', 'guest'], 'inactive');
const scores = createDictionary(['player1', 'player2', 'player3'], 0);

console.log(userNames);
console.log(scores);

{ admin: 'inactive', user: 'inactive', guest: 'inactive' }
{ player1: 0, player2: 0, player3: 0 }

Comparison

Conclusion

The for...of loop is the most straightforward method for creating dictionaries with identical values. Use Object.fromEntries() for a functional approach or create helper functions for reusability across your application.