Checking for Undefined In React

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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

In order to check for undefined, you can also use the typeof operator like

typeof nextProps.blog.content != "undefined"

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