Object is not extensible error when creating new attribute for array of objects

You probably need to copy the objects:

export const initSelect = (data) => {
 return data.map((item) => ({
     ...item,
     selected: false       
 }));
}

You can not extend item with selected property, and your array is just a shallow copy.

If you want to be able to extend, you will have to do a deep copy of your array. It may be enough with:

let newData = data.map((item) => 
    Object.assign({}, item, {selected:false})
)

const newObj = Object.assign({selected: false}, data);

data = JSON.parse(JSON.stringify(data));

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