> ## Documentation Index
> Fetch the complete documentation index at: https://docs.hoppscotch.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Pre-request scripts

> Add pre-request scripts in Hoppscotch to execute JavaScript before sending API requests. Set variables, generate tokens, and add logic.

## Scripts

Hoppscotch lets you add dynamic behavior to REST API requests. This allows you to write test suites and build requests that can contain dynamic parameters. You can add [ECMAScript](https://tc39.es/ecma262) code that executes based on events in the flow:

* Pre-request scripts are executed before a request is sent to the server.
* You can add multiple pre-request scripts to a request.
* You can add pre-request scripts to both requests saved and not saved in a collection.

<Tip>Hoppscotch will then execute the scripts along with the requests in the specified order.</Tip>

## Pre-request script

Pre-request script is a piece of code that will run before the execution of the request.

You can use the pre-request script for a pre-processing task such as:

* Setting parameters, headers.
* Adding body data.
* Adding variable values.
* Including timestamps in request headers.

## Writing pre-request scripts

Hoppscotch provides a special `pw` object containing various methods to create scripts and tests. The `pw` object is global and can be referenced by name to access methods.

For example, to set an environment variable, you can use the `pw.env.set()` method.

```javascript theme={null}
pw.env.set("variable", "value");
```

## Examples

Let us look at some examples of how you can use Hoppscotch to write pre-request scripts.

### Setting environment variables

`pw.env.set()` can be used directly for quick and convenient environment variable definition. It can be used to better organize request codes.

```javascript theme={null}
pw.env.set("baseURL", "https://httpbin.org");
pw.env.set("method", "get");
```

Go to the pre-request script tab and copy-paste the above [ECMAScript](https://tc39.es/ecma262) code as shown below:

These variables can be accessed in the request section by referencing them in double angle brackets `<<variable_name>>`. So the URL will be `<<baseURL>>/<<method>>`.

### Generating random Values to test API

Let us take a case where we need to test random test-user data available at an endpoint.

Let us use the following GET API endpoint `https://reqres.in/api/users/`.

Add `<<randomValue>>` to the endpoint URL.

```
https://reqres.in/api/users/<<randomValue>>
```

Now in the pre-request script tab add the following logic.

```javascript theme={null}
var random = Math.floor(Math.random() * 10);
pw.env.set("randomValue", random.toString());
```

The [ECMAScript](https://tc39.es/ecma262) code will assign a random number to the environment variable `randomValue` and the API will return a random user associated with the random value.

You will get a similar response as shown below:

```json theme={null}
{
  "data": {
    "id": 4,
    "email": "eve.holt@reqres.in",
    "first_name": "Eve",
    "last_name": "Holt",
    "avatar": "https://reqres.in/img/faces/4-image.jpg"
  },
  "support": {
    "url": "https://reqres.in/#support-heading",
    "text": "To keep ReqRes free, contributions towards server costs are appreciated!"
  }
}
```