Excel ROUND function in Javascript

Question

I've got a mathematical formula in Excel, which is the following:

ROUND((B2+C2)*(B55/100)/12;2)

Initial values:

• B2 = 1000
• C2 = 0
• B55 = 0,03

Results (t means time in months).

Here is my Javascript approach:

 (function _calculateRates() { var singlePayment = parseInt(1000, 10), amount = singlePayment, monthlyPayment = parseInt(0, 10), investTime = parseFloat(12), rate_a = parseFloat(0.03), rate_b = parseFloat(0.03), investment = monthlyPayment, interest = 0; for (var month = 0; month < investTime; month += 1) { investment = (month === 0)? 0: monthlyPayment; interest = Number(((amount + investment) * (rate_a / 100) / 12).toFixed(2)); amount = Number((amount + interest + investment).toFixed(2)); } console.log('Result: ', amount); })();

As one can see, the result is not correct.

Where can I find the Microsoft Excel algorithm for ROUND()?

Answer 1

In Excel =0.3/12 evaluates to 0.025 . So rounded to 2 decimals it is 0.03 .

In JavaScript var result = 0.3/12; results in 0.024999999999999998 . That .toFixed(2) is 0.02 .

Internally Excel also gets 0.024999999999999998 like all systems using IEEE Standard for Floating-Point Arithmetic (IEEE 754) . But it has the additional rule, to only take 15 digits maximum. That is 0.02499999999999 + 0.000000000000009 , which is 0.025 .

So we cannot using .toFixed in JavaScript . If we are using another method for rounding in JavaScript the this leads to the same result as in Excel .

See example using simple values:

 var result = 0.3/12; console.log(result); console.log(result.toFixed(2)); console.log(Math.floor((Math.pow(10, 2)*result)+0.5)*Math.pow(10, -2)); 

See example using your algorithm:

 (function _calculateRates() { var singlePayment = parseInt(1000, 10), amount = singlePayment, monthlyPayment = parseInt(0, 10), investTime = parseFloat(12), rate_a = parseFloat(0.03), rate_b = parseFloat(0.03), investment = monthlyPayment, interest = 0; for (var month = 0; month < investTime; month += 1) { investment = (month === 0) ? 0 : monthlyPayment; interest = Number(((amount + investment) * (rate_a / 100) / 12)); interest = Math.floor((Math.pow(10, 2)*interest)+0.5)*Math.pow(10, -2); amount = Number((amount + interest + investment)); amount = Math.floor((Math.pow(10, 2)*amount)+0.5)*Math.pow(10, -2); } console.log('Result: ', amount); })(); 

Because it is related to this question, especially why in JavaScript var result = 0.3/12; results in 0.024999999999999998 , the link to What Every Programmer Should Know About Floating-Point Arithmetic could be helpful.

Answer 2

I writed a script to round numbers in JavaScript like Excel or Google Sheets. The code is avaliable in my GitHub repository:https://github.com/marcos-nonaka/round-numbers-like-excel-in-js