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!

Answer 1

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); 

Answer 2

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); 

Answer 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); 

Answer 4

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); 

Answer 5

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

Answer 6

Yet another reducer ;)

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