# Lightning Web Security Limitations

Here are some known limitations for Lightning components running with Lightning Web Security (LWS).

## Accessors for `top` and `location` Global Properties

This limitation applies to Lightning web components (LWC) and Aura components.

The `top` property must be used with these accessors.

- `window.top`
- `globalThis.top`
- `self.top`

The `location` property must be used with these accessors.

- `window.location`
- `document.location`
- `globalThis.location`
- `self.location`
- `document.defaultView.location`

## Limitations for `window.top` and `window.location`

This limitation applies to LWC and Aura components.

Generally, it’s not recommended to override or [shadow](https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Objects/Object_prototypes#shadowing_properties) built-in global variables.

Here’s an example of an override.

```javascript
// Don't do this
window.fetch = function(...) { /* do something else */ }
```

Here’s an example of shadowing.

```javascript
// Don't do this
let location = window.location.href;
```

In LWS, properties such as `top` and `location` on the `window` global aren’t accessible to your code by default. To allow components to have access to these properties, LWS performs some code transformations behind the scenes.

For this reason it’s best to avoid shadowing of `top` and `location` properties. When shadowing those properties, your code can produce unexpected results or not work at all. For example, you can see error messages such as `Cannot access 'location' before initialization`.

One way to make sure your code is free of such patterns is to use the [ESLint `no-shadow` rule with `builtinGlobals: true`](https://eslint.org/docs/latest/rules/no-shadow#builtinglobals) option.

Rename the variables to something other than `location` or `top`. You can use `loc` instead of `location`, for example.

```javascript
// Do this instead
let loc = window.location.href;
```

## Coding Patterns Not Supported in Aura Components

These patterns produce runtime errors in Aura components:

- `async`/`await`
- dynamic import

Aura components can’t use these techniques because they’re features introduced in ES6, which isn’t supported in Aura. Use of this syntax is typically flagged by validation rules at development time.

If an Aura component uses a static resource that uses `async`/`await` or dynamic import, it’s not detected until runtime where the component encounters an error.

## Limitations for the $A Global Variable

When Aura components run with LWS, the `$A` global variable isn’t available to the `eval()` function or the `Function` constructor.

There’s no workaround for using `$A` with `eval()`.

:::tip
Using `eval()` with `force:refreshView` to refresh a view in LWC doesn’t work with LWS. Instead, use [RefreshView API](/docs/platform/lwc/guide/data-refreshview.md).
:::

### Workaround to Use $A in Functions

In Aura components, you can use dependency injection for the `Function` constructor as a workaround when LWS is enabled.

Define the function to accept an extra parameter, `$A`, which you can supply wherever it is invoked and `$A` exists on the global scope.

This example constructs a new function `fn` and specifies `$A` as one of its parameters, then invokes the function passing a reference to the accessible `$A`.

```javascript
var fn = new Function("$A", "a", "b");
fn(window.$A, 1, 2);
```

### $A is Blocked in LWC

The `$A` variable isn’t supported in Lightning web components because it’s specific to the Aura framework.

**See Also**

- [Salesforce Global Variables Blocked by LWS](/docs/platform/lightning-components-security/guide/lws-restricted-globals.md)
- [Troubleshoot Issues Related to LWS](/docs/platform/lightning-components-security/guide/lws-troubleshoot.md)