React props object data is undefined

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I'm passing an object as a prop to a component.

<SomeComponent myData={props.myData}>...</SomeComponent>

In SomeComponent, when I console.log this.props.myData, it is not undefined and contains key/val pairs, as expected.

constructor(props) {
  super(props);
  console.log(this.props.myData);
  …
}

However, when I try to access any one of this object's properties, they are all undefined.

constructor(props) {
  super(props);
  console.log(this.props.myData.title); // undefined
  …
}

Update

The following does output the value, but I'm unclear as to why, since the object is already available without setTimeout.

constructor(props) {
  super(props);
  setTimeout(() => {
    console.log(this.props.fightData.title); // "Work plain near killed us both"
  });
  …
}

What you're seeing here is very likely due to how Chrome's developer tools works. When you console.log an object, and that object updates/changes, then the output in developer tools will automatically update as well to show the new object state. I'm not entirely sure why this is the case.

So probably when you do the first example, myData is actually an empty object (due to some other part of your application), but it does get filled in later, which causes your component to update and update the Chrome developer tools.

Try running your console log like this:

console.log(JSON.stringify(this.props.myData));

And see what it outputs. This will create and print a string, so it will NOT update in the developer tools when the object changes.

When you console log this.props.myData.someVar, on an empty object, it prints undefined to the console, which is not attached to any existing object. So even when the object is updated on the component, it never updates in the console, like your first example does.

This is likely only possible for a class based component, as a functional based component is recreated every time it re-renders, but a class based component just updates its props object.

data prop in children props is undefined · Issue #71 · bvaughn/react , When I log the children component props, const Row = (props) => { console.log('-​----', props) return ( #{props.index} ) } the data prop in props is  the problem is that this.props.scope.data is undefined within the React component and if I understand this correctly, it should contain myItems? Is this a bug or am I doing something wrong here? I'm using: AngularJS v1.2.14 Latest ngReact ReactJS v0.10.0. Thank you 👍

You can try below ways to read keys and values from object

Get keys from Object

 Object.keys(this.props.myData).forEach((key) => { console.log(key + ': ' + this.props.myData[key]); });

Get values from Object

 Object.values(this.props.myData).forEach((value) => { console.log(value + ': ' + this.props.myData[value]); });

React Props Can't access Object - Programming Help, I receive an array of objects as props within the Quote component I can to go deeper to the Object it said Cannot read property 'id' of undefined I have let arr = props.value.quotes.map((data) => data).filter((data) => data.id  After all, props are only used to pass data from one component to another component React, but only from parent to child components down the component tree. React Props vs. State Passing only props from component to component doesn't make the component interactive, because nothing is there to change the props.

As @Dr_Derp suggested (and proved) it's probably a browser 'smart feature' - browser, by 'correcting' logged object value, cheats you.

For some reason props.myData doesn't have expected value in time of running constructor (and gets it just after that) - to prove that test component with static object as prop that way:

<SomeComponent myData={{title:'test'}}>...</SomeComponent>

constructor(props) {
  super(props);
  console.log(this.props.myData.title); //     should be 'test'
  …
}

stackblitz

This way it's not <SomeComponent/> fault but his parent, the way of getting and passing data to child.

Props.attribute is undefined - React - JavaScript, Props.attribute is undefined - React · Curriculum Help I would clean up your data a bit before passing it as props. I have found this saves me  The lack of default or “empty state” data can bite you another way, too: when undefined state is passed as a prop to a child component. Expanding on that example above, let’s say we extracted the list into its own component: class Quiz extends React.

I just encountered the exact same problem that you have.

using your example, I solved the issue by accessing this.props.myDatap[title] instead of this.props.myData.title

Passed state is undefined in child component, Component { render() { console.log(this.props.searchResults); return ( <div className='TrackList'> {this.props.searchResults.map(data  React is pretty flexible but it has a single strict rule: All React components must act like pure functions with respect to their props. Of course, application UIs are dynamic and change over time. In the next section, we will introduce a new concept of “state”. State allows React components to change their output over time in response to user actions, network responses, and anything else, without violating this rule.

How to pass props to components in React, Basically props are used to pass data from component to component. React props key is undefined; Pass props to Styled Components; Pass props with when passing a style object to a style attribute in React the first time. The main difference between state and props is that props are immutable. This is why the container component should define the state that can be updated and changed, while the child components should only pass data from the state using props. When we need immutable data in our component, we can just add props to reactDOM.render () function in

Connect: Extracting Data with mapStateToProps · React Redux, It receives the entire store state, and should return an object of data this pass null or undefined to connect in place of mapStateToProps . Warning: Failed prop type: The prop 'theProp' is marked as required in 'TheComponent', but its value is null. The touble is that the isRequired gets evaluated FIRST and it will never let null or undefined values through. The PR to prop-types to fix the issue is linked above if you're interested. 👍

JSX In Depth – React, If you already have props as an object, and you want to pass it in JSX, you can any other prop in that it can pass any sort of data, not just the sorts that React Conversely, if you want a value like false , true , null , or undefined to appear in  Access the navigation prop from any component useNavigation is a hook which gives access to the navigation object. It's useful when you cannot pass the navigation prop into the component directly, or don't want to pass it in case of a deeply nested child.

Comments
  • So you are saying console.log(this.props.myData) is printing an object and console.log(this.props.myData.title) is undefined if you print them at the same constructor ?
  • @MatthewBarbara Yes.
  • what is the type of myData ? is it back from mongodb query or so ?
  • Object.keys(this.props.myData).forEach((key) => { console.log(key + ': ' + this.props.myData[key]); }); you can use Object.values for values
  • I'm still getting undefined.
  • You're getting undefined from this.props.myData? I would expect you to get undefined from the scalar, because that won't update. Try putting a log in the componentDidUpdate function and see if that's getting called