How do I compare two arrays of objects to see if they have the same ids and then return another value from the matching object?

Question

So here are two sample arrays:

const sample1 = [{id: '1', name: 'jon', quantity: '5'}, {id: '2', name: 'sue', quantity: '4'}, {id: '3', name: 'greg', quantity: '7'}];

const sample2 = [{id: '1', dish: 'cheesecake'}, {id: '2', dish: 'carrot cake'}, {id: '3', dish: 'cupcake'}];

I would like to write a function to compare the ids from sample1 with the ids of sample2 . If the ids match I need to return the dish value from sample2. I tried mapping sample1 and using find() inside the loop on sample2 but could not get it to work.

Answer 1

If you want all sample2 dishes that have a matching id in sample1, the following code does the trick.

const sample1 = [{id: '1', name: 'jon', quantity: '5'}, {id: '2', name: 'sue', quantity: '4'}, {id: '3', name: 'greg', quantity: '7'}];

const sample2 = [{id: '1', dish: 'cheesecake'}, {id: '2', dish: 'carrot cake'}, {id: '3', dish: 'cupcake'}];

// Array with all Sample2 dishes that have a matching id in Sample1
const result = sample2
  .filter(sample2Entry => sample1.some(sample1Entry => sample1Entry.id === sample2Entry.id))
  .map(sample2Entry => sample2Entry.dish);

Answer 2

If u want to combine the two based on the id you can do it like this:

 const sample1 = [{id: '1', name: 'jon', quantity: '5'}, {id: '2', name: 'sue', quantity: '4'}, {id: '3', name: 'greg', quantity: '7'}]; const sample2 = [{id: '1', dish: 'cheesecake'}, {id: '2', dish: 'carrot cake'}, {id: '3', dish: 'cupcake'}]; const result = sample1.map(s1item => ({ ...s1item, dish: sample2.find(s2item => s2item.id === s1item.id) ? sample2.find(s2item => s2item.id === s1item.id).dish : null })) console.log(result)

Answer 3

You could take a Set , filter the objects and map the wanted property.

 const sample1 = [{ id: '1', name: 'jon', quantity: '5' }, { id: '2', name: 'sue', quantity: '4' }, { id: '3', name: 'greg', quantity: '7' }], sample2 = [{ id: '1', dish: 'cheesecake' }, { id: '2', dish: 'carrot cake' }, { id: '3', dish: 'cupcake' }], set1 = new Set(sample1.map(({ id }) => id)), result = sample2 .filter(({ id }) => set1.has(id)) .map(({ dish }) => dish); console.log(result);

Answer 4

You could firstly .map() your sample1 array to an array of ids . You can then build a look up table from your sample2 array which associates an array of dishes with an id key. Then using the array you made initially using .map() , you can .flatMap() over that array to get all your dishes for each given user:

 const sample1 = [{id: '1', name: 'jon', quantity: '5'}, {id: '2', name: 'sue', quantity: '4'}, {id: '3', name: 'greg', quantity: '7'}]; const sample2 = [{id: '1', dish: 'cheesecake'}, {id: '2', dish: 'carrot cake'}, {id: '3', dish: 'cupcake'}]; const sampe1map = sample1.map(({id}) => id); const lut = sample2.reduce((acc, {id, dish}) => { acc[id] = [...(acc[id] || []), dish]; return acc; }, {}); const res = sampe1map.flatMap(x => lut[x]); console.log(res);

If your ids in sample2 can be unique, you can use .map() instead of .flatMap() with a new Map() instead for your look up table:

 const sample1 = [{id: '1', name: 'jon', quantity: '5'}, {id: '2', name: 'sue', quantity: '4'}, {id: '3', name: 'greg', quantity: '7'}]; const sample2 = [{id: '1', dish: 'cheesecake'}, {id: '2', dish: 'carrot cake'}, {id: '3', dish: 'cupcake'}]; const sampe1map = sample1.map(({id}) => id); const lut = new Map(sample2.map(({id, dish}) => [id, dish])); const res = sampe1map.map(x => lut.get(x)); console.log(res);