# Building an Online IRR Calculator with React

We are done with the theory, now let's get our hands in the dirty.

In the last article, we covered the concepts of NPV (Net Present Value) and IRR (Internal Return Rate), with math insights and almost no code. Now we are going to get more serious and develop an online tool that will enable us to calculate the IRR for a given investment.

## Recap

Recall that IRR is the discount rate $$r$$ that gives us $$NPV = 0$$. In simple words, for a given initial investment and a projected cash flow over time, IRR tells us what the return rate is.

Our goal then is to compare multiple investments and analyze their IRRs in order to make better financial choices. It applies to a wide range of endeavors -- from opening a new restaurant to buying AAPL stocks. The online calculator should be, then, simple to use and quick to test multiple assumptions. Let's assess our requirements.

## Requirements

Here's a really condensed list of requirements for our app:

1. It must work online and on multiple devices (responsive);
2. The user must provide an initial amount, a period of time, and a cash flow;
3. The user may input the cash flow manually or;
4. The user may generate the cash flow automatically given a projected base cash flow and a growth rate;
5. It must be fully accessible through the keyboard;
6. The app must not accept invalid inputs.

As you can see, nothing fancy. For Requirement #4 we will need some extra math and for #5 we will need to deal with accessibility, DOM and refs.

## The App

All the code for this simulator can be found on GitHub.

Here's a simulation for one of the examples given in Making Better Investments with Math and JavaScript (Elon's proposal). Compare it with the above requirements.

Requirements #3 and #4 are covered and #6 is gracefully achieved (the button remains disabled until all inputs are present).

Let's start with the code (don't worry if you can't figure out what all lines are doing, we will revisit them soon):

import React from 'react';
import css from './index.module.css';
import { IRR } from './util';
import NumericInput from './NumericInput';
import ModeSelector from './ModeSelector';
import Result from './Result';

const MODE = { AUTO: "AUTO", MANUAL: "MANUAL" };

export default class Calculator extends React.Component {

constructor() {
super();
this.state = {
initialAmount: "",
projectedCash: "",
growthRate: "",
period: "",
IRR: "",
manualProjectedFlow: [],
projectionMethod: MODE.AUTO
};
this.investmentInput = React.createRef();
}

...

render() {
return (
<div className={css['container']}>
<h1>IRR Calculator</h1>
<main className={css['main']}>
<NumericInput
autoFocus
min={0}
label="Initial Investment"
hint="Money you need to invest upfront"
value={this.state.initialAmount}
onChange={this.onInputChange.bind(this, "initialAmount")}
ref={this.investmentInput}
/>
<NumericInput
min={1}
label="Period of Time"
hint="Total period of investment. Should be between 1 and 30"
value={this.state.period}
onChange={this.onPeriodChange.bind(this)}
/>
<ModeSelector
autoValue={MODE.AUTO}
manualValue={MODE.MANUAL}
isAutoMode={this.isAutoMode}
isManualMode={this.isManualMode}
onChange={e => this.setState({projectionMethod: e.target.value})}
/>
{this.automaticProjection}
{this.manualProjection}
<button
disabled={!this.isFormValid}
onClick={this.onClick.bind(this)}>Calculate</button>
{this.IRR}
</main>
<footer className={css['footer']}>
Created by
{' '}
<a href="https://rafaelquintanilha.com">Rafael Quintanilha</a>
</footer>
</div>
);
}
}


The constructor should be easy to follow. Note that we define our state as a list of empty values, the exception being manualProjectedFlow and projectionMode.

The first is an array which will hold the projected cash flow for a given period $t$. So, in the above .gif, manualProjectedFlow is an array of length 1 in which manualProjectedFlow === 11000. The latter determines in which mode we are: AUTO or MANUAL (defaults to AUTO).

We'll talk about this.investmentInput = React.createRef(); in a bit. But first note that apart from the mode selection (MANUAL or AUTO) all inputs are numeric. It made sense to come up with a <NumericInput /> component that we will describe next:

import React from 'react'
import css from './NumericInput.module.css';
import { uniqueId } from 'lodash';

export default class NumericInput extends React.Component {

constructor() {
super();
this.id = uniqueId("irr-");
this.input = React.createRef();
}

get hint() {
if ( !this.props.hint ) return null;
return <div className={css['hint']}>{this.props.hint}</div>;
}

focus() {
this.input.current.focus();
}

render() {
const { label, value, onChange, hint, ...rest } = this.props;
return (
<div>
<label htmlFor={this.id}>{label}</label>
<br />
<input
className={css['input']}
ref={this.input}
id={this.id}
type="number"
value={value}
onChange={onChange}
{...rest}
/>
{this.hint}
</div>
);
}
}


Couple things going on here.

First and foremost note that we use destructure assignment in order to flexibilize the accepted props for our input. We only fix some props, notably type, value and onChange.

Notice also that, in order to meet a11y standards, we generate an id by calling lodash's uniqueId. We then assign this id to both input and label, so screenreaders now can work properly.

Finally notice that we create a ref and assign it to our input. More than that, we create a class method focus() which basically focuses on the input. Why is that? Recall one of the NumericInput components of Calculator:

<NumericInput
autoFocus
min={0}
label="Initial Investment"
hint="Money you need to invest upfront"
value={this.state.initialAmount}
onChange={this.onInputChange.bind(this, "initialAmount")}
ref={this.investmentInput}
/>


The ref created in the constructor is assigned to this NumericInput, the first input and the one we want to programmatically trigger focus. By doing that, when we call this.investmentInput.current, instead of accessing the DOM element (as you may expect from refs documentation), we get a component instance and therefore we can call focus(). This technique is also explained here. Notice that this is somewhat discouraged by the React team, which advocates ref forwarding instead. While I do agree with them, I believe one can make a case for simply adding a ref to the child (for example when the child component needs to be a class component).

Let's now dive into the projection modes.

Suppose that you want to calculate the IRR for an investment which is supposed to be 30 years long. You project that it is able to yield $10,000/year and that it can grow 2.5% each year. In this case, the projected base cash inflow is $$10000$$ and the growth rate is $$0.025$$. In other words, you predict that in the first year the investment will generate $$10000$$ in cash. In the following year, $$10000 * (1 + 0.025)$$. An the end of the third year, it will be $$10000 * (1 + 0.025)^2$$ and you get the idea. How do we translate this into code? First we define a getter which only renders when on AUTO mode: get automaticProjection() { if ( this.isManualMode ) return null; return ( <React.Fragment> <NumericInput label="Projected Base Cash Inflow" value={this.state.projectedCash} onChange={this.onInputChange.bind(this, "projectedCash")} hint="How much you expect will be the base cash flow in the future" /> <NumericInput label="Growth Rate (%/period)" hint="Total period of investment. Should be between 1 and 30" value={this.state.growthRate} onChange={this.onInputChange.bind(this, "growthRate")} hint="How much your base cash flow will grow in each period" /> </React.Fragment> ); }  Which basically renders markup and assigns the correct value and onChange to each input. We then can evaluate the projected cash flow: get projectedCashFlow() { return Array(this.state.period).fill(0) .map((el, i) => this.state.projectedCash * Math.pow((1 + this.state.growthRate / 100), i)); }  The only tricky part here is creating an array of projected cash flow in a functional fashion. Instead of using a for loop (which is fine), I resorted to use Array(this.state.period).fill(0). This basically creates an array of lengths this.state.period and fills it with zeroes. The value is not particularly important, but .fill is what makes it iterable. This post is very thorough in explaining how to manipulate and create arrays in various ways. And now we can create a cash flow from three inputs: the period, the base projected cash flow and the growth rate. ## Manual Mode If you want more control over your simulation, you may simply provide values for each period individually. Look at the correspondent getter: get manualProjection() { if ( !this.isValid(this.state.period) || this.isAutoMode ) return null; return ( <React.Fragment> {Array(this.state.period).fill(0).map((el, i) => ( <NumericInput key={i} label={Cash Flow for Period${i+1}}
value={this.isValid(this.state.manualProjectedFlow[i]) ? this.state.manualProjectedFlow[i] : ""}
onChange={this.onManualCashChange.bind(this, i)}
/>
))}
</React.Fragment>
);
}


Because each period maps to a new entry in the cash flow, we necessarily need this.state.period to be a valid value. We define isValid as:

isValid(value) {
return value !== "" && !isNaN(value);
}


Again we resort to Array(this.state.period).fill(0) to generate an array of size this.state.period. When a particular cash flow entry change, we need to update this.state.manualProjectedFlow accordingly (remember we initialized it as an empty array). Here's how we do it:

onManualCashChange(i, e) {
const clone = this.state.manualProjectedFlow.concat([]);
clone[i] = parseFloat(e.target.value);
this.setState({manualProjectedFlow: clone});
}


Because in React we aim for immutability, we first clone the current array before updating it. A simple way to do it is to simply concatenate the original array with an empty one.

## Calculating the IRR

The final step is to gather all information together and evaluate the IRR. This is done in the onClick handler of the rendered button, but it will be available only if the form is valid:

get isFormValid() {
if ( this.isAutoMode ) {
return this.isValid(this.state.initialAmount)
&& this.isValid(this.state.period)
&& this.isValid(this.state.growthRate)
&& this.isValid(this.state.projectedCash);
}
else if ( this.isManualMode ) {
return this.isValid(this.state.initialAmount)
&& this.isValid(this.state.period)
&& this.state.manualProjectedFlow.length === this.state.period
&& this.state.manualProjectedFlow.reduce((acc, val) => acc && this.isValid(val), true);
}
}


For the AUTO mode we simply check that all base inputs are valid and for MANUAL mode also checks that we have a valid entry for each period. When this.isFormValid === true we can proceed:

onClick() {
const futureCashflow = this.isAutoMode ? this.projectedCashFlow : this.state.manualProjectedFlow;
const cashflow = [(-1) * this.state.initialAmount, ...futureCashflow];
this.setState({IRR: IRR(cashflow)});
}


Notice that futureCashflow is defined as the projected flow for that investment, evaluated differently in AUTO or MANUAL modes. We then append this to the initialAmount, which has its sign inverted (we are assuming nobody is paying you upfront, as unfortunate as this can be).

The complete cashflow array is then fed to the IRR function we came up with in the previous post. For the sake of this tool, though, I used a slightly different version:

export const IRR = (cashflow, initialGuess = 0.1) => {
const maxTries = 10000;
const delta = 0.001;
let guess = initialGuess;
const multiplier = NPV(cashflow, guess) > 0 ? 1 : -1;
let i = 0;
while ( i < maxTries ) {
const guessedNPV = NPV(cashflow, guess);
if ( multiplier * guessedNPV > delta ) {
guess += (multiplier * delta);
i += 1;
}
else break;
}
return i === 10000 ? "IRR has diverged" : guess;
}


Notice that now we return either guess or a friendly message if we exceeded maxTries. The computed IRR is then passed to the Result component as follow:

get IRR() {
if ( !this.state.IRR ) return null;
return <Result IRR={this.state.IRR} onResetClick={this.onResetClick.bind(this)} />;
}


Result in its turn is pretty straightforward:

import React from 'react'

export default class Result extends React.Component {

format(num) {
return parseFloat(Math.round(num * 10000) / 100).toFixed(2) + "%";
}

render() {
return (
<React.Fragment>
{isNaN(this.props.IRR)
: <div>Projected IRR is <strong>{this.format(this.props.IRR)}</strong></div>
}
<a href="#" onClick={this.props.onResetClick}>Reset</a>
</React.Fragment>
);
}
}


Notice that when this.props.IRR is not a number (for example, when it has diverged) we display the divergence message. Else, we render a nicely formatted value with 2 decimal points.

Finally, we provide a link for easily resetting all information and to start again. It's defined on Calculator:

onResetClick(e) {
// highlight-next-line
e.preventDefault();
this.setState({
initialAmount: "",
projectedCash: "",
growthRate: "",
period: "",
IRR: "",
manualProjectedFlow: []
});
// highlight-next-line
this.investmentInput.current.focus();
}


The first highlighted line is e.preventDefault, which is a simple trick to avoid appending # to the end of the URL after clicking on the anchor tag. And finally, the second highlighted line calls focus() as defined in NumericInput in order to programmatically focus on the first input after reset.

## Wrapping Up

This was a quick overview of how to create an online tool that enables users to quickly simulate investments by calculating their IRR. Again, check the full code in GitHub.

Ideas of how to improve the simulation? PRs are welcome!