Sum of previous elements from an array in Javascript

Question

I have a javascript array with these values:

fbDivHeightsTest = [280,430,408,430,408]

I need a new array with the sum of elements like this:

newArray = [280, 710, 1118, 1548, 1956].

The fbDivHeightsTest[0] + fbDivHeightsTest[1], fbDivHeightsTest[0] + fbDivHeightsTest[1] + fbDivHeightsTest[2] and so on.

I have tried like this, but the reduce function is not returning the result I expect. It returns: 280, 710, 1398, 2818, 5614

sectionsSeclector.each(function(index, item) {    
    var sum = fbDivHeights.reduce(function(a, b) { return a + b; }, 0);
    fbDivHeights.push( $(item).outerHeight() + sum);       
});

Can you please help me with this? thanks!

You are mixing up data here which might be a problem to verify right values. E.g., something like $(item).outerHeight() should not be part of your function. — thmshd
– thmshd, Commented Nov 3, 2017 at 12:02
var_nn=[];var sum = fbDivHeights.reduce(function(a, b) {return _nn.push( a + b); }); — Álvaro Touzón
– Álvaro Touzón, Commented Nov 3, 2017 at 12:02

Mihai Alexandru-Ionut · Accepted Answer · 2017-11-03 12:19:47Z

14

You can use reduce method in combination with map method.

The reduce() method applies a function against an accumulator and each element in the array (from left to right) to reduce it to a single value.

let array = [280,430,408,430,408];
array = array.map((elem, index) => array.slice(0,index + 1).reduce((a, b) => a + b));
console.log(array);

You can also use only map method.

let array = [280,430,408,430,408],sum;
array = array.map(elem => sum = (sum || 0) + elem);
console.log(array);

edited Nov 3, 2017 at 12:19

answered Nov 3, 2017 at 12:01

Mihai Alexandru-Ionut

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Comments

Nina Scholz · Accepted Answer · 2017-11-03 12:38:57Z

11

You could use a closure over the actual sum and map the incremented result.

How it works:

Assuming the function is called, then
(s => a => s += a)(0)
//                 ^  value for s
it returns
a => s += a      // s = 0
for the callback of Array#map and for every loop,
280 => s += 280  // s =  280
430 => s += 430  // s =  710
408 => s += 408  // s = 1118
430 => s += 430  // s = 1548
408 => s += 408  // s = 1956

var array =[280, 430, 408, 430, 408],
    result = array.map((s => a => s += a)(0));
    
console.log(result);

edited Nov 3, 2017 at 12:38

answered Nov 3, 2017 at 12:05

Nina Scholz

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4 Comments

Faly Over a year ago

I just don't understand

Mihai Alexandru-Ionut Over a year ago

@Faly, this code is equivalent to array.map(function (s) { return function (a) { return s += a; }; }(0));

Faly Over a year ago

Does a => s += a returns s + a ?

Nina Scholz Over a year ago

@Faly, right it's an addition assignment += operator.

Death-is-the-real-truth · Accepted Answer · 2017-11-04 06:14:43Z

3

javascript reduce() function will do the job for you easily:-

var fbDivHeightsTest = [280,430,408,430,408] 
var new_array = [];
fbDivHeightsTest.reduce(function(a,b,i) { return new_array[i] = a+b; },0);
console.log(new_array);

edited Nov 4, 2017 at 6:14

answered Nov 3, 2017 at 12:02

Death-is-the-real-truth

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Comments

Thomas · Accepted Answer · 2017-11-03 12:15:11Z

the reduce() (sometimes also called fold) method is intended to "reduce" an Array to a single value (although that single value can be an object or list)

In your example, you son't want a sum of the list, but you want to map (1:1) for each value in the initial arrray, the sum of the values to this point:

let array = [280,430,408,430,408];
let _sum = 0;
let sums = array.map(value => _sum += value);

console.log(sums);

Or as mentioned, you reduce one list to another:

let array = [280, 430, 408, 430, 408];
let sums = array.reduce((results, value, index) => {
  let previousSum = index>0? results[index-1]: 0;
  results.push(previousSum + value);
  
  return results;
}, []);

console.log(sums);

where results is an intermediate Result

Faly · Accepted Answer · 2017-11-03 12:02:26Z

1

Just use array.reduce:

var fbDivHeightsTest = [280,430,408,430,408] ;
var res = fbDivHeightsTest.reduce((m, o, i) => {
  var prev = m[i - 1];
  if (prev) {
    o = o + prev;
  } 
  m.push(o);
  return m;
}, []);
console.log(res);

answered Nov 3, 2017 at 12:02

Faly

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Comments

KooiInc · Accepted Answer · 2017-11-03 12:15:17Z

1

Yet another reducer ;)

const cumulative = [280,430,408,430,408]
       .reduce( (p, n) => p.concat((+p.slice(-1) || 0) + n), []);
 
console.log(cumulative);

answered Nov 3, 2017 at 12:15

KooiInc

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