Cannot convert undefined or null to object reactjs

Question

I was wondering about how to replace the item if it already exists inside state array and I got a great solution

It works fine but my problem is I can't add a new item if the item doesn't exist inside the array I got an error Cannot convert undefined or null to object

Like this:

add = () => {
    let newUser1 = {
      "userId": 1,
      "id": 3, // this is new id which is doesn't exist in this.state.data
      "title": "Two New",
      "body": "new data"
    }

    this.setState(prevState => {
      let newData = prevState.data;
      let user = newData.find(d => d.id === newUser1.id);
      Object.assign(user, newUser1);
      return { data: newData };
    })
  };

Try changing Object.assign(user, newUser1); to Object.assign({}, user, newUser1); — SrThompson
– SrThompson, Commented Mar 15, 2018 at 23:36

SrThompson · Accepted Answer · 2018-03-16 00:12:05Z

1

You're not adding the newUser to the data on state.

add = () => {
    let newUser1 = {
      "userId": 1,
      "id": 4,
      "title": "Two New",
      "body": "new data"
    }

    this.setState(prevState => {
      const user = prevState.data.find(d => d.id === newUser1.id);
      if (!!user) {
        //Casting user to a boolean, if find did return a truthy value you add it to the data with map
        Object.assign(user, newUser);
        const newData = prevState.data.map(d => d.id === user.id ? user : d);
        return { data: newData }
      } else {
        return { data: prevState.data.concat(newUser1)};
      }
    })
  };

edited Mar 16, 2018 at 0:12

answered Mar 15, 2018 at 23:58

SrThompson

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2 Comments

Laura delgado Over a year ago

Thank you, now I can add item but if the item already exist I can't update it as before

SrThompson Over a year ago

Yeah, my bad, you did need Object.assign :D I was just fixing it. Anyway, you should always use that method as Object.assign({}, ...etc) because JAVASCRIPT

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