React passing a prop to components return undefined

Question

I'm really confused as to why when I route to http://localhost:3000/subjects/physics of my project.

The variable gradeSelection is defined in App.js state. It is passed to subjectCards.js component via props as gradeSelection, which passes it onto a Subject.js component via props as gradeSelection.

However, this.props.gradeSelection in Subjects.js returns undefined.

Is there something I might possibly be doing wrong?

Console output:

App.js: Year 12             // correct
subjectCards.js: Year 12    // correct
Subject.js: undefined       // not correct

App.js

constructor(props) {
  super(props);
  this.state = {
    gradeSelection: "Year 12"
  };
}

render() {
  console.log("App: "+this.state.gradeSelection)
  return (
    <Route path="/subjects" render={(props)=>(<SubjectCards {...props}  gradeSelection={this.state.gradeSelection} />)} />
);


}

subjectCards.js

let display;

console.log("subjectCards.js: "+props.gradeSelection)
display = <Route path="/subjects/:subjectName" render={(props)=><Subject {...props} gradeSelection={props.gradeSelection}/>} />


return (
  display
);

Subject.js

constructor(props) {
  super(props);
  console.log("Subject.js: "+this.props.gradeSelection);  // undefined
}

Thank you!

EDIT:

When console.log(props) or console.log(this.props) in Subjects.js constructor. gradeSelection inside the console output is still undefined..

I've tried passing a string to gradeSelection in subjectCards.js and the console output was correct in returning the string in Subject.js..

display = <Route path="/subjects/:subjectName" render={(props)=><Subject {...props} gradeSelection={"props.gradeSelection"}/>} />

Answer 1

Without seeing the rest of your code, I'm going to assume that subjectCards.js is a functional component that looks like this. If it's not, could you please post the complete component?

function SubjectCards(props) {
  let display

  console.log('subjectCards.js: ' + props.gradeSelection)

  display = (
    <Route
      path="/subjects/:subjectName"
      render={props => (
        <Subject {...props} gradeSelection={props.gradeSelection} />
      )}
    />
  )

  return display
}

What I'm seeing wrong with this code in your particular use case is that on line 1, you have an argument with the name of props. If you follow the code down to line 9, you'll notice that the anonymous function call inside render also has a props argument. On line 10, you're calling props.gradeSelection which would look inside the argument found on line 9 and not the argument found on line 1, giving you undefined.

There are a couple different ways of fixing this. One way I'd recommend is destructuring your argument props on line 1.

function SubjectCards({ gradeSelection }) { // See how we went from props to {gradeSelection}
  let display

  console.log('subjectCards.js: ' + gradeSelection)

  display = (
    <Route
      path="/subjects/:subjectName"
      render={props => <Subject {...props} gradeSelection={gradeSelection} />}
    />
  )

  return display
}

You can see an example of this over at https://mo9jook5y.codesandbox.io/subjects/math

You can play around with the example here: https://codesandbox.io/s/mo9jook5y