actions should be function or object with "handler" function but "actions.actions" is {}.

import {
    ADD_COUNTER,
    ADD_TO_CART
} from './mutation-types'

export default {
    actions: {
        addCart(context, payload) {
            let oldProduct = null
            for (let item of state.cartList) {
                if (item.iid === payload.iid) {
                    oldProduct = item
                }
            }

            if (oldProduct) {
                // oldProduct.count += 1
                context.commit(ADD_COUNTER, oldProduct)
            } else {
                // payload.count = 1
                context.commit(ADD_TO_CART, payload)
            }
        }
    }
}

请问，这个问题是哪里出问题了！

你的addCart里面的东西写错了,应该这样写 addCart({commit,state},payload), 你代码的行 state都没有声明或者解构出来就使用肯定报错
https://v3.vuex.vuejs.org/zh/guide/actions.html#%E5%88%86%E5%8F%91-action 附上vuex的网址,