[Vuejs]-Merge(update) same objects inside an array

I hope you are storing data in key: value pairs Key will be item name and value will be the quantity in that case.

Before you push new entry, check the variable containing stored data whether a key in your case ‘state’ is already present. If present update value (‘payload’) alone. Else push both state and payload.

if ("key" in myObj)
{
    //Code to update payload
}
else
{
    //Code to insert key and value pair
}

More info

or try this way

function keyExists(key, search) {
        if (!search || (search.constructor !== Array && search.constructor !== Object)) {
            return false;
        }
        for (var i = 0; i < search.length; i++) {
            if (search[i] === key) {
                return true;
            }
        }
        return key in search;
    }

// How to use it:
// Searching for keys in Arrays
console.log(keyExists('apple', ['apple', 'banana', 'orange'])); // true
console.log(keyExists('fruit', ['apple', 'banana', 'orange'])); // false

// Searching for keys in Objects
console.log(keyExists('age', {'name': 'Bill', 'age': 29 })); // true
console.log(keyExists('title', {'name': 'Jason', 'age': 29 })); // false

Optimized way to do the checking

yourObjName.hasOwnProperty(key) : 
{
//Code to add payload alone 
}
? 
{
//Code to push state and payload;
}