Checking for Undefined In React

问题

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:

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.

回答1:

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"

回答2:

I was face same problem ..... And I got solution by using typeof()

if (typeof(value) !== 'undefined' || value != null) {
         console.log('Not Undefined or Not Null')
  } else {
         console.log('Undefined or Null')
}

You must have to use typeof() to identified undefined

回答3:

In case you also need to check if nextProps.blog is not undefined ; you can do that in a single if statement, like this:

if (typeof nextProps.blog !== undefined && typeof nextProps.blog.content !== undefined) { // }

And, when an undefined , empty or null value is not expected; you can make it more concise:

if (nextProps.blog && nextProps.blog.content) { // }

来源：https://stackoverflow.com/questions/40735406/checking-for-undefined-in-react

标签

reactjs

components

redux

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