Creating an array of object with cumulative sum dynamically in javascript

user_14236578 0
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I have array of objects like this and require to calculate running total(accumulate sum) dynamically for N keys.

  let a = [
  {"subject":  "Civil", "a": 100, "b":50, "c":100 },
  {"subject":  "aero",  "a": 200, "b":130,"c":100 },
  {"subject":  "tribe", "a": 100, "b":100,"c":600 },
  {"subject":  "solen", "a": 400, "b":150,"c":100},
  {"subject":  "space", "a": 100, "b":100,"c":900 }
];

Required Output as below :

[
{"subject": "Civil", "a": 100, "b": 50,  "c": 100},
{"subject": "aero",  "a": 300, "b": 180, "c": 200},
{"subject": "tribe", "a": 400, "b": 280, "c": 800},
{"subject": "solen", "a": 800, "b": 430, "c": 900},
{"subject": "space"," a": 900, "b": 530, "c": 1800}]

I tried below using MAP as below :

let b = a.map( x => ({...x,"a":a1+=x.a,"b":b1+=x.b,"c":c1+=x.c}) )

I want to achieve it dynamically without writing a, b, c.

node.js object arrays javascript
2 Answers

If you want to cumulatively sum every property other than subject, you can do so by mapping the array to new values, keeping an object reference for each running total as you go

const a = [{"subject":"Civil","a":100,"b":50,"c":100},{"subject":"aero","a":200,"b":130,"c":100},{"subject":"tribe","a":100,"b":100,"c":600},{"subject":"solen","a":400,"b":150,"c":100},{"subject":"space","a":100,"b":100,"c":900}]

const cf = {} // store running totals in here
const b = a.map(({ subject, ...props }) => ({
  subject,
  ...Object.fromEntries(Object.entries(props).map(([ key, val ]) => [
    key,
    cf[key] = (cf[key] ?? 0) + val // value is the result of the assignment
  ]))
}))

console.log(b)
.as-console-wrapper { max-height: 100% !important; }

user_2833660

You can use a combination of .map(), .slice() and .reduce() array methods as follows:

let a = [
  {"subject":  "Civil", "a": 100, "b":50, "c":100 },
  {"subject":  "aero",  "a": 200, "b":130,"c":100 },
  {"subject":  "tribe", "a": 100, "b":100,"c":600 },
  {"subject":  "solen", "a": 400, "b":150,"c":100},
  {"subject":  "space", "a": 100, "b":100,"c":900 }
];

let b = a.map((x,i) => ({
    subject:x.subject,
    a:a.slice(0,i+1).map(({a}) => a).reduce((x,y) => x + y),
    b:a.slice(0,i+1).map(({b}) => b).reduce((x,y) => x + y),
    c:a.slice(0,i+1).map(({c}) => c).reduce((x,y) => x + y)
}));

console.log( b );

Alternatively, this can be done without reference to any particular properties as follows:

let a = [
  {"subject":  "Civil", "a": 100, "b":50, "c":100, "zz":0},
  {"subject":  "aero",  "a": 200, "b":130,"c":100, "zz":40},
  {"subject":  "tribe", "a": 100, "b":100,"c":600, "zz":0},
  {"subject":  "solen", "a": 400, "b":150,"c":100, "zz":60},
  {"subject":  "space", "a": 100, "b":100,"c":900, "zz":200}
];

let b = a.map((val, i) => ({
        ...val,
        //introduce a temp property 'index'
        index: i + 1 
    }))
    .map(({
        subject,
        index,
        ...rest
    }) => ({
        subject,
        ...Object.keys(rest).reduce((obj, cur) => ({
            ...obj,
            ...{
                //use temp propterty to compute running total for every
                //property but 'subject', 
                [cur]: a.slice(0, index).map(v => v[cur]).reduce((x, y) => x + y)
            }
        }), {})
    }));   

console.log( b );

user_35589310
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