I'm a newbie in React. I have 6 divs and whenever I call foo() I want to add a number to the first div that's empty.
For example, let's say that the values of the six divs are 1,2,0,0,0,0 and when I call foo(), I want to have 1,2,3,0,0,0.
Here is what I've tried:
var index = 1;
function foo() {
let var x = document.getElementsByClassName("square") // square is the class of my div
x[index-1].innerHTML = index.toString()
index++;
}
I don't know when I should call foo(), and I don't know how should I write foo().
The "React way" is to think about this is:
What should the UI look like for the given data?
How to update the data?
Converting your problem description to this kind of thinking, we would start with an array with six values. For each of these values we are going to render a div:
const data = [0,0,0,0,0,0];
function MyComponent() {
return (
<div>
{data.map((value, i) => <div key={i}>{value}</div>)}
</div>
);
}
ReactDOM.render(<MyComponent />, document.body);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.6.3/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.6.3/umd/react-dom.production.min.js"></script>
Now that we can render the data, how are we going to change it? From your description it sounds like every time a function is called, you want change the first 0 value in the array to another value. This can easily be done with:
// Find the index of the first 0 value
const index = data.indexOf(0);
if (index > -1) {
// if it exists, update the value
data[index] = index + 1;
}
To make this work properly with React we have to do two things: Keep track of the updated data in state, so that React rerenders the component when it changes, and update the data in a way that creates a new array instead of mutating the existing array.
You are not explaining how/when the function is called, so I'm going to add a button that would trigger such a function. If the function is triggered differently then the component needs to be adjusted accordingly of course.
function update(data) {
const index = data.indexOf(0);
if (index > -1) {
data = Array.from(data); // create a copy of the array
data[index] = index + 1;
return data;
}
return data;
}
function MyComponent() {
var [data, setData] = React.useState([0,0,0,0,0,0]);
return (
<div>
{data.map((value, i) => <div key={i}>{value}</div>)}
<button onClick={() => setData(update(data))}>Update</button>
</div>
);
}
ReactDOM.render(<MyComponent />, document.body);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.0/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.0/umd/react-dom.production.min.js"></script>
You would use state to hold the value and then display the value of that variable.
If you're using functional components:
const App = () => {
const [values, setValues] = React.useState([0, 0, 0, 0, 0, 0]);
const [index, setIndex] = React.useState(0);
const foo = () => {
const tempValues = [...values];
tempValues[index] = index;
setValues(tempValues);
setIndex((index + 1) % values.length);
}
return (
<div>
{ values.map((value) => <div key={`square-${value}`}>{value}</div>) }
<button onClick={ foo }>Click me</button>
</div>
);
};
In class-based components:
constructor(props) {
super(props);
this.state = {
values: [0, 0, 0, 0, 0, 0],
index: 0
};
this.foo = this.foo.bind(this);
}
foo() {
const tempValues = [...values];
const newIndex = index + 1;
tempValues[newIndex] = newIndex;
this.setState({
values: tempValues,
index: newIndex
});
}
render() {
return (
<div>
{ values.map((value) => <div key={`square-${value}`>value</div>) }
<button onClick={ this.foo}>Click me</button>
</div>
);
}
If you need to set the innerHTML of a React component, you can try this:
return <div dangerouslySetInnerHTML={foo()} />;
the foo() here returns the value you want to post in the div.
But in my opinion, your way of thinking on this problem is wrong.
React is cool, but the logic is a bit different of common programming :D
The ideal approach would be to have the divs created by React (using its render method). Then you can pass a variable from array, which is stored in your state. You then just need to change this array within the state and it'll reflect in your view. If you need a working example, just let me know.
However, if you want to update the divs that are not created using react, then you need to use a dirty approach. I would suggest not to use react if you can't generate the view from react.
React is good to separate the concerns between the view and the data.
So the concept of state for this example is useful to store the data.
And the JSX, the React "template" language, to display the view.
I propose this solution:
import React from "react";
class Boxes extends React.Component {
state = {
divs: [1, 2, 3, 0, 0, 0]
};
add() {
// get the index of the first element equals to the condition
const index = this.state.divs.findIndex(elt => elt === 0);
// clone the array (best practice)
const newArray = [...this.state.divs];
// splice, i.e. remove the element at index AND add the new character
newArray.splice(index, 1, "X");
// update the state
// this is responsible, under the hood, to call the render method
this.setState({ divs: newArray });
}
render() {
return (
<div>
<h1>Boxes</h1>
{/* iterate over the state.divs array */}
{this.state.divs.map(function(elt, index) {
return (
<div
key={index}
style={{ border: "1px solid gray", marginBottom: 10 }}
>
{elt}
</div>
);
})}
<button onClick={() => this.add()}>Add a value</button>
</div>
);
}
}
export default Boxes;
I am writing simple application which need to rerender specified content.
My ideas about resolving the issue was to provide initializing data in constructor because something breaks react structure?But maybe helpful might be a tip how to map two dimensional array in render method. Propably here is the problem?
function Pool(props) {
return(
<p className={`${props.row}`}></p>
);
}
export default class Board extends React.Component {
constructor(props){
super(props);
this.state = {
mainArray: [],
};
}
createBoard() {
let children=[];
let array=[];
for(let i=0;i<20;i++){
children=[];
for(let j=0;j<20;j++){
children.push(<Pool key={`${i}${j}`} row={`${square1}`}/>);
}
array.push(<div key={`${i}`}>{children}</div>);
}
this.state.mainArray=array;
return this.state.mainArray;
}
startGame = () => {
let array1=[];
array1=this.state.mainArray.slice;
let i=6;
for(let j in array1[6]){
array1[6][j]=<Pool key={`${i}${j}`} row={`${square2}`}/>;
}
this.setState({mainArray: array1});
}
render() {
return (
<div className="main">
{this.createBoard()}
<button onClick={this.startGame}>Start the game!</button>
</div>
);
}
}
I am trying to change the colour of the sixth row for example.Regards
This is incorrect:
let array1=[];
array1=this.state.mainArray.slice;
mainArray.slice() is a copy, but mainArray.slice is a function.
Instead, begin with
let array1 = this.state.mainArray.slice();
I use a dictionary to store the likes of posts in every card.
constructor(props) {
super(props);
this.state = {
like_dict: {}
};
this.likeClicked = this.likeClicked.bind(this)
}
I have a like button on every card and the text denotes the number of likes.
<Button transparent onPress={() => {this.likeClicked(poid, 0)}}>
<Icon name="ios-heart-outline" style={{ color: 'black' }} />
</Button>
<Text>{this.state.like_dict[poid]}</Text>
The likedClicked function looks like this. I set the state of like_dict, but the text above won't rerender.
likeClicked(poid, liked){
var like_dict_tmp = this.state.like_dict
if (liked){
like_dict_tmp[poid] -= 1
}else{
like_dict_tmp[poid] += 1
}
this.setState({
like_dict: like_dict_tmp
})
}
One of the key principles of React is never mutate the state directly.
When you do var like_dict_tmp = this.state.like_dict, like_dict_tmp is referencing the same object in the state so altering like_dict_tmp will mutate the state directly, so the subsequent setState will not update the component.
You can create a shallow copy with var like_dict_tmp = { ...this.state.like_dict } or replace the whole function with:
this.setState((prevState) => ({
like_dict: {
...prevState,
[poid]: (prevState[poid] || 0) + (liked ? 1 : -1),
},
}));
You need to copy the state before modifying it. What you are doing is just creating a link to the state in like_dict_tmp (either use what I am doing below, or use concat with an empty array):
var like_dict_tmp = this.state.like_dict.concat([])
likeClicked(poid, liked) {
var like_dict_tmp = this.state.like_dict.slice()
if (liked) {
like_dict_tmp[poid] -= 1
} else {
like_dict_tmp[poid] += 1
}
this.setState({
like_dict: like_dict_tmp
})
}
Also, {} is an object in JS, and is not typically called a dictionary
This one is hard one. I spent some days now to solve this issue but all the solutions I tried wasn't accurate.
The thing is that I'm trying to build 'typing text effect'. This should also handle change of the sentences, ie. the first sentence is typed, then it disappears and the other sentences is starting to be typed again, and so on, it goes in the loop. The issue here is that while the first sentence is finished I would like to leave my blinker for a while, for example for 2 sec. This seems to be a perfect place to use setTimeout but all of my trials were unfortunately wrong.
I would really appreciate if you could help me! Thank you
Here's my code:
import React, { Component } from 'react';
import '../../sass/main.scss';
class TypeAnimation extends Component {
constructor(props) {
super(props);
this.state = {
sec: 0,
sec2: 0,
currentSentence: 0,
blinker: '|',
};
}
componentDidMount() {
this.textInterval = setInterval(() => {
this.setState(
prevState => ({
sec: prevState.sec !== this.props.text[0].length ? prevState.sec + 1 : 0,
currentSentence: prevState.sec === this.props.text[0].length ?
prevState.currentSentence + 1 : prevState.currentSentence,
})
);
}, 100);
}
render() {
let otherSentences;
const nextSentences = this.props.text; // here comes the array of few sentences
const currentText = nextSentences[this.state.currentSentence];
if (this.state.currentSentence !== nextSentences.length) {
otherSentences = currentText.substr(0, this.state.sec);
} else {
clearInterval(this.textInterval);
otherSentences = nextSentences[nextSentences.length - 1];
}
return (
<div>
<h2>
{otherSentences}
<span className='blinker'> {this.state.blinker} </span>
</h2>
</div>
);
}
}
export default TypeAnimation;
I'm new in ReactJS, but already got some problem I cannot resolve...
I have React component called Tree. This component must receive array of some data. Every element in this array must be rendered as a special child component called Department.
At the beginning, Tree has state {departments: []}, so, tree must be empty and it is. But then I change Tree's state, I set new array at departments, I see no child elements.
And the point is, that tree's state really updates, and function "render" is really called, and when I run thru departments, I get the right number of iterations in my cycle (checked with console.log). But still no child element appears.
My code works, I tried to render tree component with fixed departments and set this state in the constructor as initial state. Tree and child components worked fine.
So, I cannot imagine, what might be wrong.
Here is my code, my class Tree
class Tree extends React.Component {
constructor(props) {
super(props)
this.state = {
departments: []
}
this.componentDidMount = this.componentDidMount.bind(this);
}
componentDidMount() {
var xhr = new XMLHttpRequest();
xhr.open('GET', startAdress + '/tutors' + '/getAll', true);
xhr.send();
var tree = this;
xhr.onreadystatechange = function() {
if (xhr.readyState != 4) return;
if (xhr.status != 200) {
alert(xhr.status + ': ' + xhr.statusText);
} else {
var newDepts = JSON.parse(xhr.responseText);
if (Array.isArray(newDepts.content)) {
tree.setState({
departments: newDepts.content
});
}
}
};
}
render() {
var a = this;
console.log(JSON.stringify(this.state.departments));
console.log(this.state.departments.length);
return ( <div className={"Tree"}>
{
this.state.departments.forEach(function (department) {
console.log("creating branches");
return (
<Department key={department.name} department={department} />
);}
) }
</div> )
}
}
And here is my child component Department. It uses another component, called TreeLine, but I think it is not necessary to put it here.
class Department extends React.Component {
constructor(props) {
super(props)
this.state = {
}
}
render() {
console.log("department " + JSON.stringify(this.props.department));
return (
<div className={'Department'}>
<TreeLine key={this.props.department.name } classNamePostfix={"Dep"}
item={this.props.department} />
{this.props.department.items.forEach(function (item) {
return (
<TreeLine key={item.name} classNamePostfix={"Item"} item={item} />
)})
}
</div>
);
}
}
Thanks in advance!
Instead of .forEach() you should be using .map().
forEach goes through every element of the array and does something with it, but nothing is returned by forEach, even if the callback returns something. On the other hand, map creates a new array with what is being returned in the callback function as an element in that new array.
const a = [1,2,3].forEach((i) => {return <span>{i}</span>})
// a = undefined
const b = [1,2,3].map((i) => {return <span>{i}</span>})
// b = [<span>1</span>, <span>2</span>, <span>3</span>]