How export usestate, calculate, react, 3 files - reactjs

Question

I have 3 files that make up a component. I am trying to calculate a bmi result which is split into several files. When I click calculate, the page changes. I have tried to find that I am new to React in general, and these are things that are probably easy but not yet obvious to me
import React, { useState } from "react";
import style from './BMI.module.css';
import Calculator from './Calculator'
import Result from './Result'
function BMI() {
const [page, setPage] = useState(0);
const checkPage = () => {
if (page === 0) {
return (
<button className={`${style.button}`}
disabled={page === 1}
onClick={() => {
setPage((currPage) => currPage + 1);
}}>Calculate</button>
)
} else {
return (
<button className={`${style.button}`} disabled={page === 0} onClick={() => {
setPage((currPage) => currPage - 1);
}}>Back</button>
)
}
}
const PageDisplay = () => {
if (page === 0) {
return <Calculator />;
} else {
return <Result />;
}
};
return (
<div className={`${style.wrapper}`}>
<div className={`${style.box}`}>
<div className={`${style.img}`}></div>
<div className={`${style.topSection}`}>
<h1 className={`${style.title}`}>{(page === 0 ? 'BMI Calculator' : 'Result')}</h1>
</div>
<div className="body">{PageDisplay()}</div>
<div>{checkPage()}</div>
</div>
</div>
)
}
export default BMI
import React, { useState } from "react";
import style from './Calculator.module.css'
import { FontAwesomeIcon } from '#fortawesome/react-fontawesome';
import { faMars, faVenus } from '#fortawesome/free-solid-svg-icons';
const Input = ({ label, id, handleChange, name, type, placeholder }) => (
<>
<label className={`${style.label}`} htmlFor={id}>{label}</label>
<input className={`${style.input}`} type={type || "number"} id={id} name={name || id} placeholder={placeholder} onChange={(e) => handleChange(e.target.value)}></input>
<br />
</>
);
function Calculator() {
const [height, setHeight] = useState(0)
const [weight, setWeight] = useState(0)
const [age, setAge] = useState(0)
function removeSelected() {
const selectedBox = document.querySelectorAll('#head')
console.log(selectedBox.classList)
selectedBox.forEach(item => {
item.classList.remove(`${style.activeBox}`)
})
}
const handleToggle = (e) => {
if (!e.currentTarget.className.includes("activeBox")) {
removeSelected()
e.target.classList.add(`${style.activeBox}`)
} else {
e.target.classList.remove(`${style.activeBox}`)
}
};
return (
<>
<div className={`${style.content}`}>
<div className={`${style.middleSection}`}>
<h3 className={`${style.formTitle}`}>Choose your gender</h3>
<div className={`${style.genders}`}>
<div id="head" onClick={handleToggle} className={`${style.genderBox}`}>
<FontAwesomeIcon icon={faMars} className={`${style.genderIcon}`} />
<h3 className={`${style.genderBoxTitle}`}>Male</h3>
</div>
<div id="head" onClick={handleToggle} className={`${style.genderBox}`}>
<FontAwesomeIcon icon={faVenus} className={`${style.genderIcon}`} />
<h3 className={`${style.genderBoxTitle}`}>Female</h3>
</div>
</div>
</div>
<div className={`${style.bottomSection}`}>
<Input handleChange={setWeight} placeholder='Weight' label='Your weight (kg)'>{weight}</Input>
<Input handleChange={setHeight} placeholder='Height' label='Your height (cm)'>{height}</Input>
<Input handleChange={setAge} placeholder='Age' label='Your age'>{age}</Input>
</div>
</div>
</>
)
}
export default Calculator
import React, { useState } from 'react'
import style from './Result.module.css'
function Result() {
const [bmiScore, setBmiScore] = useState(0)
const [bmiDesc, setBmiDesc] = useState('')
return (
<div className={`${style.content}`}>
<div className={`${style.img}`}></div>
<div className={`${style.descriptions}`}>
<p className={`${style.bmiScoreDesc}`}>Your BMI is <span className={`${style.bmiScoreNumber}`}>{bmiScore}</span>, indication your weight is in the <span className={`${style.bmiScoreDesc}`}>{bmiDesc}</span> category for adults of your height</p>
<p className={`${style.descriptionBottom}`}>Maintaining a healthy weight may reduce the risk of chronic diseases associated with overweight and obesity.</p>
</div>
</div>
)
}
export default Result
So when i click Calculate which i have in the BMI.js take inputs value from Calculator.js calculate the score and put into paragraph in Result.js

Answer 1

What you will want to do in this case is at the top level, in BMI, that's where your bmiScore state variable should live. You can pass it to both toe Calculator (where it will be calculated) and the Result (where it will be displayed) via props, like so:
<Calculator bmi={bmiScore} /> and <Result bmi={bmiScore} />
Then the method signatures in each will look like:
function Calculator({ bmi }) and function Result({ bmi })
And you can use that bmi variable in both.
This is all because React passes variables down, not up. The only way to share across components is if a parent component holds the variable.