从对象数组获取唯一值及其数量

Question

So I get a JSON request that sends an array of objects with two properties, I need to extract the unique values and their quantities.

This is the JSON that is being sent via Postman:

[
    {"name": "First value", "amount": 2},
    {"name": "Second value", "amount": 4},
    {"name": "First value", "amount": 6}  
]

I need to return a JSON response with the unique values and their totals added up:

The object should look like this:

{
   "First value": 8,
   "Second value": 4
}

Answer 1

You can use reduce()

 const arr = [ {"name": "First value", "amount": 2}, {"name": "Second value", "amount": 4}, {"name": "First value", "amount": 6} ] const res = arr.reduce((ac, {name, amount}) => { ac[name] = ac[name] || 0; ac[name] += amount; return ac; },{}) console.log(res) 

Explanation:

First of all we are initializing ac to an empty object {} . See the line

arr.reduce((ac, {name, amount}) => {...}
                ^^^^^^^^^^^^^^ 

The highlighted part is called Object destructuring. It will get the property name and amount out the current object through which we are iterating and make it independent variables.

See the line

ac[name] = ac[name] || 0;

Now this line is checking if ac[name] doesn't exist on the ac object then it will be undefined so undefined || 0 undefined || 0 will be evaluate to 0 . If it will have a value the value of ac[name] will remain as previous.

See the third line:

ac[name] += amount;

This line will add the amount to already value of ac[name]

At last we return ac so that it will be become initial value of ac for next iteration.

Answer 2

You could also simply use Array.forEach as it is for the most part easier to understand and follow:

 let arr = [{ "name": "First value", "amount": 2 }, { "name": "Second value", "amount": 4 }, { "name": "First value", "amount": 6 } ], obj={} arr.forEach(({name, amount}) => { obj[name] = obj[name] || 0 obj[name] += amount }) console.log(obj) 

The main difference here is that our accumulator (the obj ) is defined outside rather than "internally" as an argument to the Array.reduce function.