Add example for using global variables

docs/sources/k6/next/examples/use-global-variables.md

@@ -0,0 +1,83 @@
+---
+title: 'Use global variables in k6'
+description: 'Learn how and when to use global variables in k6.'
+weight: 13
+---
+
+# Use global variables in k6
+
+A global variable is a variable that can be accessed from any scope. In the context of k6, global variables can be used to persist and pass information across iterations of a test run, and must be declared in the [`init` stage](https://grafana.com/docs/k6/<K6_VERSION>/using-k6/test-lifecycle/#the-init-stage).
+
+A common use case is when you need to extract a value from a page, such as a CSRF or an API token, and reuse that across iterations of your test.
+
+There are multiple ways to use global variables in JavaScript, each with benefits and drawbacks.
+
+## Option 1: use `import` and `export`
+
+The first option is to declare a global variables object in your main script and export it.
+
+```js
+import { doSomething, doSomethingElse } from './imported-script.js';
+
+export const VARS = {};
+
+export default function () {
+  VARS.prop = 'prop1';
+
+  doSomething();
+
+  doSomethingElse();
+  console.log(JSON.stringify(VARS));
+}
+```
+
+You can then import the variable in your script, and use it across functions or alter the values:
+
+```js
+import { VARS } from './main.js';
+
+export function doSomething() {
+  console.log(JSON.stringify(VARS));
+}
+
+export function doSomethingElse() {
+  VARS.prop2 = 'prop2';
+}
+```
+
+The advantage of using this method is the fact that you need to explicitly use the `import` and `export` statements. That can make it easier when working in a team, so other developers can understand your script logic and see where the variables and values came from.
+
+## Option 2: use `globalThis`
+
+The second option is to use the `globalThis` JavaScript object, which is available in any JavaScript environment, not just k6.
+
+For example, in your main script you can add a property to `globalThis`:
+
+```js
+import { doSomething, doSomethingElse } from './imported-script.js';
+
+globalThis.VARS = {};
+
+export default function () {
+  globalThis.VARS.prop = 'prop1';
+
+  doSomething();
+
+  doSomethingElse();
+  console.log(JSON.stringify(globalThis.VARS));
+}
+```
+
+And, similarly to the previous option, you can use access it from anywhere in that script or in imported scripts as well:
+
+```js
+export function doSomething() {
+  console.log(JSON.stringify(globalThis.VARS));
+}
+
+export function doSomethingElse() {
+  globalThis.VARS.prop2 = 'prop2';
+}
+```
+
+The advantage to using the `globalThis` object is that you don't need to `import` or `export` a global variable. The downside is that, depending on how complex or extensive your test scripts are, using the `globalThis` variable can make your script more verbose, and cause unintended behavior if you're not careful about how you're using and setting global variables.