In my lightning component I have sObject list which I render as data table. Some of the cells/fields are editable and onChange of those fields I recalculate other fields.
In my lightning component Helper method (called via controller when field value changes), I call another helper method and pass the field name as String which I need to update. Is there a way to dynamically reference that field and update it's value?
 

//Helper Method 1
calculateSegmentsAmounts : function(component, event, helper) {
        
        var prepayRecord = component.get("v.prepaymentRecord");
        var segmentsList = component.get("v.prepaymentSegmentsList");
        var calculatedTotalPercentage = 0;
        var calculatedTotalPrincipal = 0;
        var calculatedTotalInterest = 0;
        var calculatedTotalPremium = 0;
        var calculatedTotalBreakage = 0;
        var calculatedTotalOther = 0;
        
        for(var i=0; i < segmentsList.length; i++) {
            
            var percentage = 0;
            var principal = 0;
            
            if(segmentsList[i].Selected__c) {
                                    
				percentage = segmentsList[i].PAR_Percentage__c / 100;
				principal = percentage * prepayRecord.Native_Principal__c;                
                
                segmentsList[i].Interest__c = percentage * prepayRecord.Native_Interest__c;
                segmentsList[i].Premium__c = percentage * prepayRecord.Native_Premium__c;
                segmentsList[i].Breakage__c = percentage * prepayRecord.Native_Breakage__c;
                segmentsList[i].Other__c = percentage * prepayRecord.Native_Other__c;
                segmentsList[i].Total_Segment__c = principal + segmentsList[i].Interest__c + segmentsList[i].Premium__c + segmentsList[i].Breakage__c + segmentsList[i].Other__c;
            }
            percentage = percentage * 100;
            calculatedTotalPercentage += percentage;
            segmentsList[i].Principal__c = principal;
               
            calculatedTotalPrincipal += segmentsList[i].Principal__c;
            calculatedTotalInterest += segmentsList[i].Interest__c;
            calculatedTotalPremium += segmentsList[i].Premium__c;
            calculatedTotalBreakage += segmentsList[i].Breakage__c;
            calculatedTotalOther += segmentsList[i].Other__c;
        } // Loop ends
		
		// Update attribute value because it will be referred in below Helper methods
        component.set("v.prepaymentSegmentsList", segmentsList);

        var reCalcTotal = false;
        if(calculatedTotalPercentage == 100.00000000000) {
            if(calculatedTotalPrincipal != 0 && calculatedTotalPrincipal != prepayRecord.Native_Principal__c) {
                helper.adjustExtraAmount(component, event, helper, prepayRecord.Native_Principal__c, calculatedTotalPrincipal, "Principal__c"); // Passing Field name as String
                reCalcTotal = true;
            }
            if(calculatedTotalInterest != 0 && calculatedTotalInterest != prepayRecord.Native_Interest__c) {
                helper.adjustExtraAmount(component, event, helper, prepayRecord.Native_Interest__c, calculatedTotalInterest, "Interest__c"); // Passing Field name as String
                reCalcTotal = true;
            }
            if(calculatedTotalPremium != 0 && calculatedTotalPremium != prepayRecord.Native_Premium__c) {
                helper.adjustExtraAmount(component, event, helper, prepayRecord.Native_Premium__c, calculatedTotalPremium, "Premium__c"); // Passing Field name as String
                reCalcTotal = true;
            }
            if(calculatedTotalBreakage != 0 && calculatedTotalBreakage != prepayRecord.Native_Breakage__c) {
                helper.adjustExtraAmount(component, event, helper, prepayRecord.Native_Breakage__c, calculatedTotalBreakage, "Breakage__c"); // Passing Field name as String
                reCalcTotal = true;
            }
            if(calculatedTotalOther != 0 && calculatedTotalOther != prepayRecord.Native_Other__c) {
                helper.adjustExtraAmount(component, event, helper, prepayRecord.Native_Other__c, calculatedTotalOther, "Other__c"); // Passing Field name as String
                reCalcTotal = true;
            }
            if(reCalcTotal) {
                helper.reCalculateSegmentsRowTotal();
            }
        }
        
        component.set("v.prepaymentRecord", prepayRecord);
        component.set("v.prepaymentSegmentsList", segmentsList);
    },
	
	//Helper Method 2
	adjustExtraAmount : function(component, event, helper, nativeAmount, calculatedTotal, fieldName) {
    	
        var segmentsList = component.get("v.prepaymentSegmentsList");
        var multiplier = 100;
        var difference = (Math.round(multiplier * nativeAmount) - Math.round(multiplier * calculatedTotal)) / multiplier;    
        var absdiff = Math.abs(difference);
        var calculatedMoreThanActual;
        if(nativeAmount < calculatedTotal) {
            calculatedMoreThanActual = true;
        }
        else {
            calculatedMoreThanActual = false;
        }
        if (absdiff >= 0.01 && absdiff < 1) {
    
            var affliatedInvIndex = helper.getLargestaffiliatedINVIndex(component, event, helper, fieldName);  // Another Helper to which fieldName is passed 
            if (affliatedInvIndex == -1) {
                affliatedInvIndex = helper.getLargestSegmentIndex(component, event, helper, fieldName); // Another Helper to which fieldName is passed 
            }       
            var nonAffliatedInvIndex = helper.getLowestNonAffiliatedINVIndex(component, event, helper, fieldName); // Another Helper to which fieldName is passed 
            if (nonAffliatedInvIndex == -1) {
                nonAffliatedInvIndex = helper.getLowestSegmentIndex(component, event, helper, fieldName); // Another Helper to which fieldName is passed 
            }
            var runOnce = false;
            console.log('>>>> Recalculating ' + fieldName);
            for(var i=0; i < segmentsList.length; i++) {
                
                if(segmentsList[i].Selected__c) {
                    
                    var oldValue = 0;
                    oldValue = segmentsList[i].fieldName;   // Need to dynamically fetch the value
                    if(calculatedMoreThanActual) {
                        if (i == nonAffliatedInvIndex && !runOnce) {
                            var delta = oldValue + difference;          
                            segmentsList[i].fieldName = delta;     // Need to dynamically Update the value                       
                            runOnce = true;
                        }   
                    }
                    else {
                        if (i == affliatedInvIndex && !runOnce ) {
                            var delta = oldValue + difference;
                            segmentsList[i].fieldName = delta; 	// Need to dynamically Update the value
                            runOnce = true;
                        }
                    }               
                }
            }
        }
        component.set("v.prepaymentSegmentsList", segmentsList);
    },