React: Variable refers to new Date() can not be a child element.

[krave1986]

I am following a code snippet from the book React Quickly.
And I can not run the code after being compiled via babel.
My code is like:

class DateTimeNow extends React.Component{
    render(){
        let dateTimeNow = new Date();
        return <span>Current date and time is {dateTimeNow}.</span>;
    }
}
ReactDOM.render(
    <DateTimeNow/>,
    document.getElementById('content')
);

The error message reported by browser is:
Uncaught Invariant Violation: Objects are not valid as a React child (found: Mon Oct 17 2016 09:12:00 GMT+0800 (CST)). If you meant to render a collection of children, use an array instead or wrap the object using createFragment(object) from the React add-ons. Check the render method ofDateTimeNow.

Could you please help to answer why it will cause error?
Thank you very much!

Do you want to request a _feature_ or report a _bug_?

What is the current behavior?
error message: Uncaught Invariant Violation: Objects are not valid as a React child (found: Mon Oct 17 2016 09:12:00 GMT+0800 (CST)). If you meant to render a collection of children, use an array instead or wrap the object using createFragment(object) from the React add-ons. Check the render method of DateTimeNow.

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template: https://jsfiddle.net/reactjs/69z2wepo/).

What is the expected behavior?
span tag displays current time
Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?
React 15.3.2

[colw]

The short answer is you need to call a toString() method on your dateTimeNow variable, as it is a date _object_, and not a string. i.e. one of the 'Objects' the error message is referring to.

return <span>Current date and time is {dateTimeNow.toUTCString()}.</span>;

That said, the error message is initially confusing since it simply renders the date as a string. If for example you replace {dateTimeNow} with an object such as { {a: 1} }, the error message makes a little more sense:

Uncaught Error: Objects are not valid as a React child (found: object with keys {a}).

[gaearon]

https://github.com/facebook/react/issues/7988#issuecomment-254148427 is correct.

Dates are objects in JS, but your render output may only contain React elements, strings, and numbers. Formatting a string with any toString()-like method will work.

You can find a list of such methods on MDN.

Cheers!

[ipsips]

why can't React do calling .toString() for us automatically a lá string interpolation? Too much overhead?