How to filter a map in Javascript

We can filter a map in JavaScript in three distinct ways: forEach(), filter(), and for...of loops.

Filtering with forEach() Method:

The forEach() method is a fundamental approach to iterate over a Map and selectively delete elements based on a specified condition.

const map1 = new Map([
  ['num1', 1],
  ['num2', 2],
  ['num3', 3],
]);

map1.forEach((value, key) => {
  if (value > 1) {
    map1.delete(key);
  }
});

// Result: {'num1' => 1}
console.log(map1);

This method, driven by a callback function, operates on each key-value pair of the Map, checking conditions and modifying the Map accordingly.

Alternative Approach: Using filter() Method:

Converting a Map to an array and using the filter() method provides an alternative perspective on Map filtering.

const map1 = new Map([
  ['num1', 1],
  ['num2', 2],
  ['num3', 3],
]);

const filtered = new Map(
  Array.from(map1).filter(([_key, value]) => value > 1),
);

// Result: {'num2' => 2, 'num3' => 3}
console.log(filtered);

This approach involves array conversion, destructuring assignment, and filtering using the filter() method.

Explore with for...of Loop:

Filtering a Map using a for...of loop offers a classic yet readable solution.

const map1 = new Map([
  ['num1', 1],
  ['num2', 2],
  ['num3', 3],
]);

const filtered = new Map(
  Array.from(map1).filter(([_key, value]) => value > 1),
);

// Result: {'num2' => 2, 'num3' => 3}
console.log(filtered);

This loop, designed for iterable objects, simplifies the process of iterating over a Map and selectively deleting elements.

Test Your Understanding:

Before we conclude, let’s reinforce your knowledge with a practice question:

Given a Map originalMap containing key-value pairs, write a function filterMapGreaterThanN that filters out all entries where the corresponding values are greater than a given threshold n. The function should return a new Map with the filtered entries.

// Sample Map
const originalMap = new Map([
  ['num1', 5],
  ['num2', 8],
  ['num3', 3],
  ['num4', 12],
]);

// Function to filter Map entries
function filterMapGreaterThanN(inputMap, n) {
  // < < < Your code here > > >
}

// Test the function
const filteredMap = filterMapGreaterThanN(originalMap, 7);

// Expected Result: {'num2' => 8, 'num4' => 12}
console.log(filteredMap);

function filterMapGreaterThanN(inputMap, n) {
  // Use the filter() method along with Array.from() to create a new Map
  const filteredMap = new Map(
    Array.from(inputMap).filter(([_key, value]) => value > n)
  );

  return filteredMap;
}

Conclusion:

In this technical guide, we explored three detailed methods for filtering a Map in JavaScript: forEach(), filter(), and for...of loops. As you master these techniques, remember that efficient Map filtering is a crucial skill in your JavaScript toolkit.