# Initializing an object property if assigning to it fails

If you initialize a variable like `let applications = {}` and assign a property of `applications` with an array, like as in `applications['foo'].push(1)` it'll throw an error because `foo` is not *yet* a property of `applications`.

In Chrome it throws the following error :

> caught TypeError: Cannot read properties of undefined (reading 'push')

In Firefox it throws :

> Uncaught TypeError: applications.foo is undefined

In Safari it throws :

> undefined is not an object (evaluating 'applications\[property\].push'

But at times we would want to push to an array of a property in `applications` like `applications['foo'].push(5)` where we don't know beforehand if `applications.foo` is defined or not.

In that case, we define it as `applications.foo = []`.

But, if we don't have a list of properties beforehand to mass assign to `applications`, if the data is coming from an AJAX request, then we can initialize the property using a try-catch block.

```javascript
try
{
    applications[property].push(response[i]['version']);
}
catch(e)
{
    if (
        // Chrome
        e.message.includes("Cannot read properties of undefined") ||
        // FireFox
        e.message.includes("applications[property] is undefined") ||
        // Safari
        e.message.includes("undefined is not an object (evaluating 'applications[property].push'")
        )
    {
        applications[property] = [response[i]['version']];
    }
    else
    {
        console.error("error = ", e.message);
    }
}
```

The catch here is to check for two error messages - one for Chrome and one for Firefox.

**UPDATE:** A guy who works on the Chromium code-base for his company, The Browser Company (Arc.net) mentioned about using the optional chaining operator (`?.`)which was introduced in ES2020 - so now the entire try-catch block can be replaced with just this :

```javascript
applications[property]?.push(response[i]['version']);
```

Thanks @[Vivek Galatage](@vivekgalatage).

Here is the full example source: [https://gist.github.com/anjanesh/fda4de82fe22f82ba0db5caf08308968](https://gist.github.com/anjanesh/fda4de82fe22f82ba0db5caf08308968)

%[https://gist.github.com/anjanesh/fda4de82fe22f82ba0db5caf08308968]