Check if object key exists within object

Question

I'm trying to take some data from an existing object and group it into a new one. The problem I am having is checking if the object key exists so I can either create a new one, or append data to an existing one.

I've found a few similar questions but none of the answers worked so I'm a bit stuck. It always ends up finding it doesn't exist and creating duplicate keys.

I have the following code, where xxx is where I need to check if the key exists:

var groups = [];    

for (var i=0; i<something.length; i++) {

    var group_key = 'group_'+something[i].group_id;

    if (xxx) {

        // New group

        var group_details = {};

        group_details[group_key] = {
                group_name: something[i].group_name,
                items:  [
                    { 'name': something[i].name }
                ]
        };
        groups.push(group_details);

    } else {

        // Existing group

        groups[group_key].items.push({
            'name': something[i].name
        });

    }

}

The something I am passing in, is pretty simple, basically in the form of:

[
    {
        group_id: 3,
        group_name: 'Group 3',
        name: 'Cat'
    },
    {
        group_id: 3,
        group_name: 'Group 3',
        name: 'Horse'
    },
    {
        group_id: 5,
        group_name: 'Group 5',
        name: 'Orange'
    }
]

Answer 1

The best way to achieve this would be to rely on the fact that the in operator returns a boolean value that indicates if the key is present in the object.

var o = {k: 0};

console.log('k' in o); //true

But this isin't your only issue, you do not have any lookup object that allows you to check if the key is already present or not. Instead of using an array, use a plain object.

var groups = {};

Then instead of groups.push(...), do groups[group_key] = group_details;

Then you can check if the group exist by doing if (group_key in groups) {}

Answer 2

I have run into this pattern a lot, and what I end up doing is:

if (object[key]) {
    //exists
} else {
    // Does not exist
}

so I think in your case it will be:

if (groups[group_key]) {
    // Exists
} else {
    // Does not exist 
}

Answer 3

let data = {key: 'John'};
console.log( data.hasOwnProperty('key') );

Answer 4

You should use the in operator

key in object     //true
!(key in object)  //false

And undefined can not be used

obj["key"] !== undefined //false, but the key in the object

For more information, please look at Checking if a key exists in a JavaScript object?