Loop through array of objects, sum array elements and put result in last element

Question

I have an array of objects as follows. Each object has an array P2 inside it.

let input = [ 
     { "P2": [ 6, 6, 0 ] },
     { "P2": [ 3, 2, 0 ] },
     { "P2": [ 5, 1, 0 ] },
]

I want the desired result. as you can see, i am adding the elements and putting the result in the last element. for e.g. adding 6+6+0 and putting the result in last element as 12. Similarly for other objects inside the array. So the final output will look like this.

   [ 
     { "P2": [ 6, 6, 0, 12 ] },
     { "P2": [ 3, 2, 0, 5 ] },
     { "P2": [ 5, 1, 0, 6 ] },
   ]

I am able to achieve this with a simple loop and reduce function as shown below.

input.forEach(arrayItem => {
  let sum = arrayItem.P2.reduce((partialSum, a) => partialSum + a, 0);
  arrayItem.P2.push(sum)
});

console.log(input)

Its fine till here. but it gets really interesting when i have dynamic input. for e.g. inside for static P2 values, i can have anything else. For e.g. shown below are the 2 examples of dynamic input.

let input = [ 
     { "P1": [ 6, 6, 0 ] },
     { "P2": [ 3, 2, 0 ] },
     { "P3": [ 5, 1, 0 ] },
]

OR

let input = [ 
     { "P2": [ 3, 3, 3, 2 ] },
     { "P2": [ 7, 7, 7, 4 ] },
     { "Total": [ 5, 1, 4, 6 ] },
]

Can someone let me know how to modify my code if there is dynamic property names like this.

Answer 1

you can iterate over each object in the array and dynamically access the property name using Object.keys() or Object.entries().

let input = [ 
    { "P1": [ 6, 6, 0 ] },
    { "P2": [ 3, 2, 0 ] },
    { "P3": [ 5, 1, 0 ] },
];

input.forEach(arrayItem => {
    let propertyName = Object.keys(arrayItem)[0];
    let values = arrayItem[propertyName];
    let sum = values.reduce((partialSum, value) => partialSum + value, 0);
    values.push(sum);
});

console.log(input);

Using your second example

let input = [ 
     { "P2": [ 3, 3, 3, 2 ] },
     { "P2": [ 7, 7, 7, 4 ] },
     { "Total": [ 5, 1, 4, 6 ] },
]

input.forEach(arrayItem => {
    let propertyName = Object.keys(arrayItem)[0];
    let values = arrayItem[propertyName];
    let sum = values.reduce((partialSum, value) => partialSum + value, 0);
    values.push(sum);
});

console.log(input);

Answer 2

Just iterate and push a sum as the last element to the sub arrays:

let input = [ 
     { "P1": [ 6, 6, 0 ] },
     { "P2": [ 3, 2, 0 ] },
     { "P3": [ 5, 1, 0 ] },
]

input.forEach(item => {
  const [arr] = Object.values(item);
  arr.push(arr.reduce((r, n) => r+n));
});
console.log(input);

Answer 3

You may also use the array map method as follows:

let input = [ 
     { "P2": [ 6, 6, 0 ] },
     { "P2": [ 3, 2, 0 ] },
     { "P2": [ 5, 1, 0 ] },
];

input = input.map(({P2}) => ({P2:[...P2,P2.reduce((a,c) => a+c,0)]}));

console.log( input );

Now for this, you can use the array map method and Object.keys() and Object.values():

let input = [ 
     { "P1": [ 6, 6, 0 ] },
     { "P2": [ 3, 2, 0 ] },
     { "P3": [ 5, 1, 0 ] },
];

input = input.map(o => ({
    [Object.keys(o)[0]]: [
        ...Object.values(o)[0],
        Object.values(o)[0].reduce((a,c) => a+c,0)
    ]})
);

console.log( input );

And similarly for this one:

let input = [ 
     { "P2": [ 3, 3, 3, 2 ] },
     { "P2": [ 7, 7, 7, 4 ] },
     { "Total": [ 5, 1, 4, 6 ] },
];

input = input.map(o => ({
    [Object.keys(o)[0]]: [
        ...Object.values(o)[0],
        Object.values(o)[0].reduce((a,c) => a+c,0)
    ]})
);
        
        
console.log( input );

Or, if you choose to use Object.entries() and Object.fromEntries:

let input = [ 
     { "P2": [ 3, 3, 3, 2 ] },
     { "P2": [ 7, 7, 7, 4 ] },
     { "Total": [ 5, 1, 4, 6 ] },
];

input = input.map(o => Object.fromEntries(
        Object.entries(o).map(
            ([key,values]) => [
                key,
                [...values,values.reduce((a,c) => a+c,0)]
            ]
        )
    )
);
        
        
console.log( input );

Answer 4

The problem with your code is that you are using arrayItem.P2 in the function and expecting it to access every object inside the array. It will only work for 'P2' and won't work for any other keys like 'P1', 'P3' and like that. So you need to access the array without explicitly mentioning the key names. For this you can use Object.keys method. In your case you should be using Object.keys(arrayItem) to access the array inside the objects.

input.forEach((arrayItem) => {
  let key = Object.keys(arrayItem);
  const sum = arrayItem[key]
    .reduce((a, b) => {
      a += b;
      return a;
    });
  arrayItem[key].push(sum);
});

console.log(input)

<script>
  let input = [{
      "P1": [6, 6, 0]
    },
    {
      "P2": [3, 2, 0]
    },
    {
      "P3": [5, 1, 0]
    },
  ]
</script>