Without immer, with immer

Without immer, constructing immutables is error-prone:

increaseOrderQuantity(item) {
    const itemIndex = this.state.data.indexOf(item);

    this.setState(({data}) => ({
        data: [
            ...data.slice(0, itemIndex),
            {
                ...item,
                quantity: item.quantity + 1
            },
            ...data.slice(itemIndex + 1) // have made a mistake of forgetting to include + 1
        ]
    }));
}

With immer, less error-prone:

import produce from 'immer';

.
.
.


increaseOrderQuantity(item) {
    const itemIndex = this.state.data.indexOf(item);
    
    this.setState(produce(draft => {
        draft.data[itemIndex].quantity++;
    }));
}