lost9123193 lost9123193 - 14 days ago 6
React JSX Question

Checking for Undefined In React

I have a scenario where I'm passing data from a reducer into my react state.

data:

{
"id": 1,
"title": "Test",
"content": {
"body": "sdfsdf"
"image": "http://example.com"
}
}


Using componentWillRecieveProps, this works perfectly for retrieving the title.

componentWillReceiveProps(nextProps) {
this.setState({
title: nextProps.blog.title,
})
}


However, I'm having difficulty retrieving the nested fields. When I do this:

componentWillReceiveProps(nextProps) {
console.log("new title is", nextProps.blog.title);
console.log("new body content is", nextProps.blog.content["body"]);
this.setState({
title: nextProps.blog.title,
body: nextProps.blog.content["body"]
})
}


I get this error:

enter image description here

The error of an undefined body goes away after I click the debugger and the content is loaded. Is there anyway I can combat this issue?

I tried to check for undefined like this:

if (typeof nextProps.blog.content["body"] != 'undefined'){


But this doesn't work either and I believe it's because the blog is undefined.

Answer

What you can do is check whether you props is defined initially or not by checking if nextProps.blog.content is undefined or not since your body is nested inside it like

componentWillReceiveProps(nextProps) {

    if(nextProps.blog.content !== undefined && nextProps.blog.title !== undefined) {
       console.log("new title is", nextProps.blog.title);
       console.log("new body content is", nextProps.blog.content["body"]);
       this.setState({
           title: nextProps.blog.title,
           body: nextProps.blog.content["body"]
       })
    }
}

You need not use type to check for undefined, just the strict operator !== which compares the value by their type as well as value

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