spread operator (...) is creating extra fields in array in es6

Question

I wanted to embed a new key/value pair in the respective indexed array of objects based on an onChange event.

However, it is done correctly but adding extra elements in the array.

Original array of objects:

0:{data: {…}}
1:{data: {…}}
2:{data: {…}}
3:{data: {…}}
4:{data: {…}}

Achieved result:

0:{data: {…}}
1:{data: {…}}
2:{data: {…}, origin: "UK"}
3:{data: {…}, origin: "UK"}
4:{data: {…}}
5:"UK"
6:"UK"

Intended result:

0:{data: {…}}
1:{data: {…}}
2:{data: {…}, origin: "UK"}
3:{data: {…}, origin: "UK"}
4:{data: {…}}

Below is my code doing it in a loop:

render: (rowData, indexes) => {
          return (
            <SelectField
              id={`origin-${indexes.rowIndex}`}
              defaultValue="US"
              style={{ position: 'absolute' }}
              onChange={text => {
                this.setState(
                  {
                    generalPermitSelectedVehicles: [
                      ...generalPermitSelectedVehicles,
                      (generalPermitSelectedVehicles[
                        indexes.rowIndex
                      ].origin = text),
                    ],
                  },
                  () => {
                    console.log({
                      generalPermitSelectedVehicles: this.state
                        .generalPermitSelectedVehicles,
                    });
                  },
                );
              }}
              menuItems={[
                {
                  label: 'America',
                  value: 'US',
                },
                {
                  label: 'United Kingdom',
                  value: 'UK',
                },
                {
                  label: 'Oman',
                  value: 'Oman',
                },
              ]}
            />
          );
        },

[`...` is not an operator!](https://stackoverflow.com/questions/37151966/what-is-spreadelement-in-ecmascript-documentation-is-it-the-same-as-spread-oper/37152508#37152508) — Felix Kling, Feb 17 '18 at 16:00
What is it called, actually? What's its purpose? Can you elaborate it? We can learn from your statement. Thanks — Owais, Feb 19 '18 at 08:32

Mayank Shukla · Accepted Answer · 2018-02-16T07:26:48.613

Write it like this:

this.setState(prevState => {
   let data = [...prevState.generalPermitSelectedVehicles];
   data[indexes.rowIndex].origin = text;
   return {generalPermitSelectedVehicles: data};
})

Why its failing in your case?

Because when you do:

[...arr, (arr[index].origin=10)]

It will do two things, first it will update the value of origin at that index, second it will add 10 (returned 10 from ()) at the end of array also.

Check this snippet:

let arr = [{a:1}, {a:2}, {a:3}];
arr = [...arr, (arr[1].a=500)];  //500 will get added in the last

console.log('new value', arr);

Suggestion: Use updater function (prevState) because next state (value) of generalPermitSelectedVehicles is dependent on previous value.

Check the DOC for more details about setState updater function.

Shubham Khatri · Answer 2 · 2018-02-16T07:25:09.700

You need to update the original state and not append it. You are not using spread operator correctly. Also make use of functional setState when you want to update state based on prevState. You would need to do

 this.setState(
              prevState => ({
                generalPermitSelectedVehicles: [
                  ...prevState.generalPermitSelectedVehicles.slice(0, index.rowIndex),
                  {...prevState.generalPermitSelectedVehicles[
                    indexes.rowIndex
                  ], origin: text},
                  ...prevState.generalPermitSelectedVehicles.slice(index.rowIndex + 1)
                ],
              },
              () => {
                console.log({
                  generalPermitSelectedVehicles: this.state
                    .generalPermitSelectedVehicles,
                });
              },
            );

The error in your approach is that you are appending the updated state after spreading the original state, you need to update the existing instead.

Also check this answer on how to update nested state

spread operator (...) is creating extra fields in array in es6

2 Answers2