old way: function accumulate ( arr ) { let result = [] let sum = 1 0 for ( let i = 0 ; i < arr . length ; i ++) { sum += arr [ i ] result . push ( sum ) } return result } in functional programming no mutation - don't mutate data pure function - no side effects const so we need to use filter, map, concat, reduce, closure: a closure is a function that references variables in the outer scope from its inner scope. function accumulate ( arr ) { let total = 10 return (() => arr . map (( num ) => { total += num return total }) ) } console . log ( sum ([ 1 , 2 , 3 ])()) => [ 11, 13, 16 ]
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