How can I efficiently copy the entries from one Map to another Map where the target Map has multiple reference variables?

Question

I have found a way to copy the entries from one Map to another Map where the target Map has multiple reference variables but I suspect it is not optimal. Is there a shorter/more efficient way?

const mapFrom = new Map([[1, 'from']]); const mapTo = new Map([[1, 'to']]); const refMapTo = mapTo; mapTo.clear(); for (const [key, value] of mapFrom) mapTo.set(key, value); if (mapTo.get(1) === mapFrom.get(1) && mapTo === refMapTo) { console.log('this code works but can I avoid the `for...of` iteration?'); }

@IvanYonkov because I suspect mapFrom will grow very large and browser performance could be degraded — user2309803
– user2309803, Commented Feb 1 at 9:18
There's no way to copy (at later stage) without looping through the values. Either mapTo must be a reference to mapFrom from the beginning or you need to have this loop one way or another — Ivan Yonkov
– Ivan Yonkov, Commented Feb 1 at 9:31

rozsazoltan · Accepted Answer · 2025-02-01 09:55:08Z

I don't think there's anything particularly wrong with using for...of. If you really want to avoid it, you could try using forEach, but opinions on this are divided.

Here are a few related topics from earlier.

Loop (for each) over an array in JavaScript - StackOverflow
Loops- forEach, for, for....of, for...in - StackOverflow
Why should forEach be preferred over regular iterators? - StackOverflow
Should one use for-of or forEach when iterating through an array? - StackOverflow
What is the most efficient way to deep clone an object in JavaScript? - StackOverflow

I don't know the goal of the task. If the goal is simply copying, using the constructor or forEach seems like a good solution. Among the mentioned posts, there are a few that go quite deep into the topic, and I can only recommend reading all of them.

// Constants for map size and test iterations const MAP_SIZE = 100000; const TEST_RUNS = 100; // Create a map with MAP_SIZE entries const mapFrom = new Map(); for (let i = 0; i < MAP_SIZE; i++) { mapFrom.set(i, `value_${i}`); } // Function to run tests TEST_RUNS times and calculate average, min, and max function test(testName, testFunction) { const times = []; // Run the test TEST_RUNS times for (let i = 0; i < TEST_RUNS; i++) { const start = performance.now(); testFunction(); const end = performance.now(); times.push(end - start); } // Calculate average, min, and max const avg = times.reduce((a, b) => a + b, 0) / times.length; const min = Math.min(...times); const max = Math.max(...times); // Log the results console.log(`${testName}:`); console.log(` Average Time: ${avg.toFixed(2)} ms`); console.log(` Min Time: ${min.toFixed(2)} ms`); console.log(` Max Time: ${max.toFixed(2)} ms`); } // 1. Using Map constructor to copy test('Map Constructor', () => { const mapToConstructor = new Map(mapFrom); }); // 2. Using for...of loop to copy test('for...of loop', () => { const mapToForOf = new Map(); for (const [key, value] of mapFrom) { mapToForOf.set(key, value); } }); // 3. Using forEach to copy test('forEach', () => { const mapToForEach = new Map(); mapFrom.forEach((value, key) => { mapToForEach.set(key, value); }); });

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