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Most performant way to copy a Map into an existing Map

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4

I want to overwrite Map b in the snippet below with the contents of Map a. I am currently clearing the contents of b and then setting the entries one by one. What is a more efficient way (less garbage, faster, etc.) of writing the copyMap function?

const a = new Map([[1, 2], [2, 10], [3, 20]]);
const b = new Map([[1,5]]);

// I want to copy a into b, at the end b should look like
// b = new Map([[1, 2], [2, 10], [3, 20]]);

console.log(a,b);

function copyMap(srcMap, destMap) 
 destMap.clear();
 
 // Maybe this loop could be done in a single call somehow?
 for(let item of srcMap) 
 destMap.set(item[0], item[1]);
 

copyMap(a, b);

// a and b should have same entries
console.log('a: ',[...a.entries()], 'b: ', [...b.entries()]);

javascript dictionary ecmascript-6 clone

share|improve this question

edited Mar 27 at 14:22

Cristy

asked Mar 27 at 13:24

CristyCristy

14.8k2020 gold badges6969 silver badges117117 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Do you want to mutate b, or would you be fine with creating a new Map?
  
  – Bergi
  Mar 27 at 13:50
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I don't think copyMap could get any simpler than what you did. (Maybe use destructuring for item, but that's just syntax).
  
  – Bergi
  Mar 27 at 13:52
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  My main goal is to reduce garbage generation, as it is for a high-performance app where GC pauses can ruin user experience and I want to re-use as much of the already instantiated objects as possible. I do think (from previous experiences) that changing an existing object is faster and generates less garbage than creating a new one.
  
  – Cristy
  Mar 27 at 14:18
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  No, Maps don't have a bulk copy method. The only method that takes an iterable is the constructor. I don't think it does pre-allocate the memory to the expected size, but you could still try to do b = new Map(a).
  
  – Bergi
  Mar 27 at 18:47
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I have multiple app instances (tens or hundreds) running at 60FPS on a single thread, every several frames a new a is received (different for each instance) and I have to save it/copy it into b. In my specific case, both a and b have (mostly) the same keys (but different values for those keys), each time the copy function is used, so probably it's better not to clear the map before, but only set/overwrite current keys.
  
  – Cristy
  Mar 27 at 20:53
  

  
  
  
  

 | 
show 7 more comments

4

I want to overwrite Map b in the snippet below with the contents of Map a. I am currently clearing the contents of b and then setting the entries one by one. What is a more efficient way (less garbage, faster, etc.) of writing the copyMap function?

const a = new Map([[1, 2], [2, 10], [3, 20]]);
const b = new Map([[1,5]]);

// I want to copy a into b, at the end b should look like
// b = new Map([[1, 2], [2, 10], [3, 20]]);

console.log(a,b);

function copyMap(srcMap, destMap) 
 destMap.clear();
 
 // Maybe this loop could be done in a single call somehow?
 for(let item of srcMap) 
 destMap.set(item[0], item[1]);
 

copyMap(a, b);

// a and b should have same entries
console.log('a: ',[...a.entries()], 'b: ', [...b.entries()]);

javascript dictionary ecmascript-6 clone

share|improve this question

edited Mar 27 at 14:22

Cristy

asked Mar 27 at 13:24

CristyCristy

14.8k2020 gold badges6969 silver badges117117 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Do you want to mutate b, or would you be fine with creating a new Map?
  
  – Bergi
  Mar 27 at 13:50
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I don't think copyMap could get any simpler than what you did. (Maybe use destructuring for item, but that's just syntax).
  
  – Bergi
  Mar 27 at 13:52
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  My main goal is to reduce garbage generation, as it is for a high-performance app where GC pauses can ruin user experience and I want to re-use as much of the already instantiated objects as possible. I do think (from previous experiences) that changing an existing object is faster and generates less garbage than creating a new one.
  
  – Cristy
  Mar 27 at 14:18
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  No, Maps don't have a bulk copy method. The only method that takes an iterable is the constructor. I don't think it does pre-allocate the memory to the expected size, but you could still try to do b = new Map(a).
  
  – Bergi
  Mar 27 at 18:47
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  I have multiple app instances (tens or hundreds) running at 60FPS on a single thread, every several frames a new a is received (different for each instance) and I have to save it/copy it into b. In my specific case, both a and b have (mostly) the same keys (but different values for those keys), each time the copy function is used, so probably it's better not to clear the map before, but only set/overwrite current keys.
  
  – Cristy
  Mar 27 at 20:53
  

  
  
  
  

 | 
show 7 more comments

I want to overwrite Map b in the snippet below with the contents of Map a. I am currently clearing the contents of b and then setting the entries one by one. What is a more efficient way (less garbage, faster, etc.) of writing the copyMap function?

const a = new Map([[1, 2], [2, 10], [3, 20]]);
const b = new Map([[1,5]]);

// I want to copy a into b, at the end b should look like
// b = new Map([[1, 2], [2, 10], [3, 20]]);

console.log(a,b);

function copyMap(srcMap, destMap) 
 destMap.clear();
 
 // Maybe this loop could be done in a single call somehow?
 for(let item of srcMap) 
 destMap.set(item[0], item[1]);
 

copyMap(a, b);

// a and b should have same entries
console.log('a: ',[...a.entries()], 'b: ', [...b.entries()]);

javascript dictionary ecmascript-6 clone

share|improve this question

edited Mar 27 at 14:22

Cristy

asked Mar 27 at 13:24

CristyCristy

14.8k2020 gold badges6969 silver badges117117 bronze badges

const a = new Map([[1, 2], [2, 10], [3, 20]]);
const b = new Map([[1,5]]);

// I want to copy a into b, at the end b should look like
// b = new Map([[1, 2], [2, 10], [3, 20]]);

console.log(a,b);

function copyMap(srcMap, destMap) 
 destMap.clear();
 
 // Maybe this loop could be done in a single call somehow?
 for(let item of srcMap) 
 destMap.set(item[0], item[1]);
 

copyMap(a, b);

// a and b should have same entries
console.log('a: ',[...a.entries()], 'b: ', [...b.entries()]);

const a = new Map([[1, 2], [2, 10], [3, 20]]);
const b = new Map([[1,5]]);

// I want to copy a into b, at the end b should look like
// b = new Map([[1, 2], [2, 10], [3, 20]]);

console.log(a,b);

function copyMap(srcMap, destMap) 
 destMap.clear();
 
 // Maybe this loop could be done in a single call somehow?
 for(let item of srcMap) 
 destMap.set(item[0], item[1]);
 

copyMap(a, b);

// a and b should have same entries
console.log('a: ',[...a.entries()], 'b: ', [...b.entries()]);

const a = new Map([[1, 2], [2, 10], [3, 20]]);
const b = new Map([[1,5]]);

// I want to copy a into b, at the end b should look like
// b = new Map([[1, 2], [2, 10], [3, 20]]);

console.log(a,b);

function copyMap(srcMap, destMap) 
 destMap.clear();
 
 // Maybe this loop could be done in a single call somehow?
 for(let item of srcMap) 
 destMap.set(item[0], item[1]);
 

copyMap(a, b);

// a and b should have same entries
console.log('a: ',[...a.entries()], 'b: ', [...b.entries()]);

share|improve this question

edited Mar 27 at 14:22

Cristy

asked Mar 27 at 13:24

CristyCristy

14.8k2020 gold badges6969 silver badges117117 bronze badges

share|improve this question

edited Mar 27 at 14:22

Cristy

asked Mar 27 at 13:24

CristyCristy

14.8k2020 gold badges6969 silver badges117117 bronze badges

asked Mar 27 at 13:24

CristyCristy

14.8k2020 gold badges6969 silver badges117117 bronze badges

asked Mar 27 at 13:24

CristyCristy

14.8k2020 gold badges6969 silver badges117117 bronze badges

1 Answer
1

active

oldest

votes

0

you can jsut do

b=new Map(a)

and you need to loose the const declaration for b

share|improve this answer

answered Mar 27 at 23:16

jonathan Heindljonathan Heindl

63422 silver badges1313 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  But this way previous b Map will become garbage, and I want to avoid creating garbage.
  
  – Cristy
  Mar 28 at 9:08
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  then why not copy it in a new variable ?
  
  – jonathan Heindl
  Mar 28 at 9:11
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What do you mean, I just explained above? I do the copyMap operation hundreds of times a second, if I create a new Map each time it will do a lot of allocations and create a lot of garbage.
  
  – Cristy
  Mar 28 at 9:12
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  See Memory Management to better understand the garbage collection issue that OP is trying to solve here.
  
  – benvc
  Mar 29 at 2:01
  

  
  
  
  

add a comment | 

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0

you can jsut do

b=new Map(a)

and you need to loose the const declaration for b

share|improve this answer

answered Mar 27 at 23:16

jonathan Heindljonathan Heindl

63422 silver badges1313 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  But this way previous b Map will become garbage, and I want to avoid creating garbage.
  
  – Cristy
  Mar 28 at 9:08
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  then why not copy it in a new variable ?
  
  – jonathan Heindl
  Mar 28 at 9:11
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What do you mean, I just explained above? I do the copyMap operation hundreds of times a second, if I create a new Map each time it will do a lot of allocations and create a lot of garbage.
  
  – Cristy
  Mar 28 at 9:12
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  See Memory Management to better understand the garbage collection issue that OP is trying to solve here.
  
  – benvc
  Mar 29 at 2:01
  

  
  
  
  

add a comment | 

0

you can jsut do

b=new Map(a)

and you need to loose the const declaration for b

share|improve this answer

answered Mar 27 at 23:16

jonathan Heindljonathan Heindl

63422 silver badges1313 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  But this way previous b Map will become garbage, and I want to avoid creating garbage.
  
  – Cristy
  Mar 28 at 9:08
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  then why not copy it in a new variable ?
  
  – jonathan Heindl
  Mar 28 at 9:11
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  What do you mean, I just explained above? I do the copyMap operation hundreds of times a second, if I create a new Map each time it will do a lot of allocations and create a lot of garbage.
  
  – Cristy
  Mar 28 at 9:12
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  See Memory Management to better understand the garbage collection issue that OP is trying to solve here.
  
  – benvc
  Mar 29 at 2:01
  

  
  
  
  

add a comment | 

you can jsut do

b=new Map(a)

and you need to loose the const declaration for b

share|improve this answer

answered Mar 27 at 23:16

jonathan Heindljonathan Heindl

63422 silver badges1313 bronze badges

b=new Map(a)

share|improve this answer

answered Mar 27 at 23:16

jonathan Heindljonathan Heindl

63422 silver badges1313 bronze badges

answered Mar 27 at 23:16

jonathan Heindljonathan Heindl

63422 silver badges1313 bronze badges

answered Mar 27 at 23:16

jonathan Heindljonathan Heindl

63422 silver badges1313 bronze badges