How can you check if a javascript map has a duplicate key

base
October 20, 2024
133 views
4 votes
4 Answers

I’m working with a Map in JavaScript and need to check that it doesn’t contain duplicate keys. The map I’m working with is below.

const colors = new Map([
    [1, 'blue'],
    [2, 'orange'],
    [1, 'pink'], // This duplicate key should trigger an error
    [3, 'yellow'],
]);

In this situation because the number 1 is used two times as a key that should throw an error.

I cannot modify the colors variable directly. Instead, I need to make a function that checks for duplicate keys in the Map and throw an error if a duplicate is found.

This is what I currently have. The problem is that when you use a duplicate key in the map it overrides the previous key and value so it’s as if a duplicate key is never in the map.

const colors = new Map([
    [1, 'blue'],
    [2, 'orange'],
    [1, 'pink'], // This duplicate key should trigger an error
    [3, 'yellow'],
]);

const iterator = colors.keys();

console.log(iterator.next().value);
// output: 1

console.log(iterator.next().value);
// output: 2

console.log(iterator.next().value);
// output: 3

Tags: javascript

Answers

I have addd checkForDuplicateKeys
the Set (seenKeys) keep track of keys.
if a duplicate key is found it throws an error.

function checkForDuplicateKeys(entries) {
    const seenKeys = new Set(); 
    
    for (const [key, value] of entries) {
        if (seenKeys.has(key)) {
            throw new Error(`duplicate key found: ${key}`);
        }
        seenKeys.add(key); // add the key to the set
    }
}

const colorEntries = [
    [1, 'blue'],
    [2, 'orange'],
    [1, 'pink'], // This duplicate key should trigger an error
    [3, 'yellow'],
];

try {
    checkForDuplicateKeys(colorEntries); 
    //if no error
    const colors = new Map(colorEntries); 
    console.log(colors);
} catch (error) {
    console.error(error.message); 
}

My suggestion:

const entries = [
  [1, 'blue'],
  [2, 'orange'],
  [1, 'pink'],
  [3, 'yellow'],
];

function checkDuplicateKeys(entries) {
  const keySet = new Set();
  const duplicates = [];

  entries.forEach(([key, value], index) => {
    if (keySet.has(key)) {
      duplicates.push({
        key,
        index
      });
    } else {
      keySet.add(key);
    }
  });

  if (duplicates.length > 0) {
    const keys = duplicates.map(d => d.key).join(', ');
    const errorMessage = `Duplicate key(s) found: ${keys}.`;

    throw new Error(errorMessage);
  }
}

try {
  checkDuplicateKeys(entries);
  const colors = new Map(entries);
  console.log('Map created successfully:', colors);
} catch (error) {
  console.error(error.message);
}

In case of duplicate key, the function will throw an error displaying that key (in the example above: 1).

const values = [
    [1, "blue"],
    [2, "orange"],
    [1, "pink"], // This duplicate key should trigger an error
    [3, "yellow"],
];
const colors = new Map(
    values.map((value) => {
        if (values.filter((element) => value[0] == element[0]).length == 1) {
            return value;
        } else {
            throw new Error(` key ${value[0]} is duplicate `);
        }
    })
);

I’d go with comparison of Array length and Set size, since creating a Set with duplicate values will reduce the existing ones. The magic happens in hasDuplicateEntries

const colorEntriesWithDuplicate = [
  [1, 'yellow'],
  [2, 'blue'],
  [1, 'green'],
  [3, 'red']
];

const colorEntriesWithoutDuplicate = [
  [1, 'yellow'],
  [2, 'blue'],
  [3, 'red']
];

const hasDuplicateEntries = (entries) => {
  return entries.length !== new Set(entries.map(([key, val]) => key)).size;
}


console.log(hasDuplicateEntries(colorEntriesWithDuplicate));

console.log(hasDuplicateEntries(colorEntriesWithoutDuplicate));

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