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How can I check if element does not exist in a React test?

I want to test that on initial render of the parent component, there are no child components rendered in the document.
On every press of the button, the parent component generates a child component within it. On init, the child component array is empty. I therefore expect my child componet test-id to be null on initial render, when I render my parent component.
Parent component:
const ParentComponent = () => {
const [childComponentsArray, setChildComponentsArray] = useState([]);
const createChildComponent = () => {
const objToAdd = {
// Generate uuid for each component
uuid: uuid()
};
setChildComponentsArray([...childComponentsArray, objToAdd]);
};
return (
<div>
{childComponentsArray.length > 0 && <div>
{childComponentsArray.map(() => {
return <div className={'child-item'}>
<ChildComponent />
</div>;
})}
</div>}
<ButtonContainer variant={'secondary'} label={'+ ' + component.addLabel}
onClick={() => createChildComponent()}
/>
</div>
);
};
Child component:
const ChildComponent = () => {
return (
<div data-testid={'childComponentTestId'}>
<p> I'm in child component! </p>
</div
)
}
Unit test:
test('on render, no child items are visible', () => {
render(
<RecoilRoot>
<ParentComponent />
</RecoilRoot>
)
expect(screen.getByTestId('childComponentTestId')).toBeNull();
});
When executing my test I get the following error in my unit test:
TestingLibraryElementError: Unable to find an element by: [data-testid="childComponentTestId"]
I find this error a bit of a paradox, since that is exactly what I want it to be.
note
Passing the data-testid as a prop does not help.
Using .not.toBeInDocument() does not work.
Using .toBeUndefined() does not work either.

You should use queryByTestIt as it returns null if object is not found.
See more on the documentation site.

Can I store a component in a variable and push children to it after it has been created?

Let's say that I have this component:
const Test = ({ children, ...rest }) => {
return <>{children}</>
};
export default Test;
I am wondering if it is possible to create a variable that holds the component like this:
const test = <Test></Test>;
And then loop over some data and push children to the test variable on every iteration.

if you don't have the data yet, then all you have to do is conditionally render your component when you do have the data.
{ data ? (<Test>{data.map(...)}</Test>) : <SomeOtherComponent /> /* or null */}
or
{ data ? <>{data.map((x) => <Test>{x}</Test>)}</> : <SomeOtherComponent /> /* or null */}
depending on what you want achieve, i didn't fully understand your question
i.e. if you have the data you need, render the component, rendering the children as you see fit, otherwise render some other component (or null, to render nothing)

Yeap, try that pattern:
const test = (children) => <Test>{children}</Test>;
and usage
<>
{[1,2,3].map(el=>test(el))}
</>
[Edited]
const TestComp = ({children}) => <Test>{children}</Test>;
<>
{[1,2,3].map(el=>(<TestComp>{el}</TestComp>))}
</>

Can't pass useState() 'set' function to grand child

I'm having issues trying to get my useState variable to work. I create the state in my grandparent then pass it into my parent. Here's a simplified version of my code:
export function Grandparent(){
return(
<div>
const [selectedID, setSelectedID] = useState("0")
<Parent setSelectedID2={setSelectedID} .../> //(elipses just mean that I'm passing other params too)
<div />
)}
Parent:
const Parent = ({setSelectedID2 ...}) => {
return(
<div>
{setSelectedID2("5")} //works
<Child setSelectedID3={setSelectedID2} />
</div>
)
}
From the parent I can use 'setSelectedID2' like a function and can change the state. However, when I try to use it in the child component below I get an error stating 'setSelectedID3' is not a function. I'm pretty new to react so I'm not sure if I'm completely missing something. Why can I use the 'set' function in parent but not child when they're getting passed the same way?
Child:
const Child = ({setSelectedID3 ...}) => {
return(
<div >
{setSelectedID3("10")} //results in error
</div>
);
};

In React you make your calculations within the components/functions (it's the js part) and then what you return from them is JSX (it's the html part).
export function Grandparent(){
const [selectedID, setSelectedID] = useState("0");
return(
<div>
<Parent setSelectedID2={setSelectedID} .../> //(elipses just mean that I'm passing other params too)
<div />
)}
You can also use (but not define!) some js variables in JSX, as long as they are "renderable" by JSX (they are not Objects - look for React console warnings).
That's your React.101 :)

Here's a working example with everything you have listed here. Props are passed and the function is called in each.
You don't need to name your props 1,2,3.., they are scoped to the function so it's fine if they are the same.
I moved useState and function calls above the return statement, because that's where that logic should go in a component. The jsx is only used for logic dealing with your display/output.
https://codesandbox.io/s/stupefied-tree-uiqw5?file=/src/App.js
Also, I created a working example with a onClick since that's what you will be doing.
https://codesandbox.io/s/compassionate-violet-dt897?file=/src/App.js
import React, { useState } from "react";
export default function App() {
return <Grandparent />;
}
const Grandparent = () => {
const [selectedID, setSelectedID] = useState("0");
return (
<div>
{selectedID}
<Parent setSelectedID={setSelectedID} selectedID={selectedID} />
</div>
);
};
const Parent = ({ selectedID, setSelectedID }) => {
setSelectedID("5");
return (
<div>
{selectedID}
<Child setSelectedID={setSelectedID} selectedID={selectedID} />
</div>
);
};
const Child = ({ selectedID, setSelectedID }) => {
setSelectedID("10");
return <div>{selectedID}</div>;
};
output
10
10
10

const [selectedID, setSelectedID] = useState("0")
should be outside return

React component state not updating with passed props

I have created an Accordion component which has data(object) and expanded(boolean) as props.
expanded props is used to set the expanded/collapsed state of this component passed as a prop.
const DeltaAccordion = ({ index, data, expanded = true }) => {
Accordion component also has an internal state
const [isExpanded, setIsExpanded] = useState(expanded);
which is used for expanding/collapsing the accordion.
Below is my complete component
Accordion.jsx
import React, { useState } from "react";
// styles
import styles from "./index.module.scss";
const Accordion = ({ index, data, expanded = true }) => {
// state
const [isExpanded, setIsExpanded] = useState(expanded);
console.log(data.name, `prop-val==${expanded}`, `inner-state==${isExpanded}`);
return (
<div
className={`${styles.container} caption ${isExpanded && styles.expanded}`}
>
<div className={styles.header} onClick={() => setIsExpanded(!isExpanded)}>
<div>{data.name}</div>
<div>Click</div>
</div>
<div className={styles.content}>
{data.newValue && (
<div className={styles.newValue}>
<span>{data.newValue}</span>
</div>
)}
{data.oldValue && (
<div className={styles.oldValue}>
<span>{data.oldValue}</span>
</div>
)}
</div>
</div>
);
};
export default Accordion;
The parent component has a list of data and it loops through the list to create an accordion for each data item.
App.js file
{dataList.map((data, index) => (
<Accordion data={data} expanded={!collpaseAll} index={1} />
))}
Here goes problem
There is also a button in my App.js file which is for either expanding/collapsing all the accordions together.
But when I pass its value as prop expanded to the accordion component then this value is not getting applied to the internal isExpanded state of the accordion component.
Here is live running code at codesandbox: https://codesandbox.io/s/goofy-nobel-qfxm1?file=/src/App.js:635-745

Inside the Accordion
const [isExpanded, setIsExpanded] = useState(expanded);
This line will take first time(on first render) value. To assign it next time(rerender) value you need to add a effect
useEffect(() => {
setIsExpanded(expanded);
}, [expanded]);
And in your case, you can use the props expanded directly inside Accordion, you dont need to take it in local state.

Value not getting passed from child to parent component in React

As you can see in the two components below, i want to delete the recipes(in app component) from a button click in the panelcomponent,
i have a method in app to delete the recipe, and a prop(onclick) send to child panelcomponent. Panel then gets the index from the map of recipes, and after the button click it executes the handleDelet method to send the index back to parent. but No this is not working !
class App extends React.Component {
state={
addRecipe:{recipeName:"",ingredients:[]},
recipes:[{recipeName:"Apple",ingredients:["apple","onion","spice"]},
{recipeName:"Apple",ingredients:["apple","onion","spice"]},
{recipeName:"Apple",ingredients:["apple","onion","spice"]}]
}
handleDelete = (index) => {
let recipes = this.state.recipes.slice();
recipes.splice(index,1); //deleting the index value from recipe
this.setState({recipes}) //setting the state to new value
console.log(index,recipes)
}
render() {
return (
<div className="container">
<PanelComponent recipes={this.state.recipes} onClick={()=>this.handleDelete(index)}/>
<ModalComponent />
</div>
);
}
}
class PanelComponent extends React.Component {
handleDelete = (index) => {
this.props.onClick(index); //sending index to parent after click
console.log(index)
}
render() {
return (
<PanelGroup accordion>
{this.props.recipes.map( (recipe,index) => {
return(
<Panel eventKey={index} key={index}>
<Panel.Heading>
<Panel.Title toggle>{recipe.recipeName}</Panel.Title>
</Panel.Heading>
<Panel.Body collapsible>
<ListGroup>
{recipe.ingredients.map((ingredient)=>{
return(<ListGroupItem>{ingredient}</ListGroupItem>);
})}
</ListGroup>
<Button bsStyle="danger" onClick={()=>this.handleDelete(index)}>Delete</Button>
<EditModalComponent />
</Panel.Body>
</Panel>
);
})}
</PanelGroup>
);
}
}

Thea actual error in your code is that while using arrow function in the onClick in parent, you are passing the wrong parameter, instead of {()=>this.handleDelete(index)} what you should write is
{(value)=>this.handleDelete(value)}, However, that also not necessary and you could simple write {this.handleDelete} in App since your handleDelete function is already binded and it received the values from the Child component.
render() {
return (
<div className="container">
<PanelComponent recipes={this.state.recipes} onClick={(value)=>this.handleDelete(value)}/>
<ModalComponent />
</div>
);
}
The difference in writing {()=>this.handleDelete(index)} vs {(value)=>this.handleDelete(value)} is that in the first case, you are explicitly passing the index that you get from the map function in your App component while in the second case, the value passed from the child component when you execute this.props.onClick(value) is being provided to the handleDelete function.

you are sending the function wrongly as props. you are sending the result of the function as props rather than the function itself
class App extends React.Component {
state={
addRecipe:{recipeName:"",ingredients:[]},
recipes:[{recipeName:"Apple",ingredients:["apple","onion","spice"]},
{recipeName:"Apple",ingredients:["apple","onion","spice"]},
{recipeName:"Apple",ingredients:["apple","onion","spice"]}]
}
handleDelete = (index) => {
let recipes = this.state.recipes.slice();
recipes.splice(index,1); //deleting the index value from recipe
this.setState({recipes}) //setting the state to new value
console.log(index,recipes)
}
render() {
return (
<div className="container">
//change here
<PanelComponent recipes={this.state.recipes} onClick={this.handleDelete}/>
<ModalComponent />
</div>
);
}
}