|
| 1 | +--- |
| 2 | +title: Runtime Execution Context |
| 3 | +slug: en/docs/concepts/runtime-execution-context |
| 4 | +sidebar: |
| 5 | + order: 3 |
| 6 | +--- |
| 7 | + |
| 8 | +In JavaScript, the global object varies depending on the execution environment. In the browser, the global object is `window`, which includes properties like `document`. In Node.js, the global object is `global`. This object is specific to Node.js and is not available in browsers. |
| 9 | + |
| 10 | +For example, you cannot access `process.env` in a browser environment because it is specific to Node.js. Similarly, calling `document.createElement` on a server does not work, as it is intended for the client-side. Attempting either will result in a runtime error. |
| 11 | + |
| 12 | +In Open Data Capture, code runs on both the client and server. You should therefore not access browser-specific APIs, like `document` or `window` unless you are within functions named `render`, which execute exclusively on the client. For those familiar with Next.js, think of these functions as rendering a component with a "use client" directive within a React Server Component. |
| 13 | + |
| 14 | +## Example of Problematic Instrument |
| 15 | + |
| 16 | +```ts |
| 17 | +import { defineInstrument } from '/runtime/v1/@opendatacapture/runtime-core'; |
| 18 | +import { z } from '/runtime/v1/[email protected]'; |
| 19 | + |
| 20 | +const button = document.createElement('button'); |
| 21 | +button.textContent = 'Submit Instrument'; |
| 22 | +document.body.appendChild(button); |
| 23 | + |
| 24 | +export default defineInstrument({ |
| 25 | + // ... |
| 26 | + kind: 'INTERACTIVE', |
| 27 | + content: { |
| 28 | + render(done) { |
| 29 | + button.addEventListener('click', () => { |
| 30 | + done({ message: 'Hello World' }); |
| 31 | + }); |
| 32 | + } |
| 33 | + } |
| 34 | + // ... |
| 35 | +}); |
| 36 | +``` |
| 37 | + |
| 38 | +## Fixing This Instrument |
| 39 | + |
| 40 | +To fix the instrument, there are two options: |
| 41 | + |
| 42 | +1. Move the browser-specific code directly within the render function |
| 43 | +2. Wrap the browser-specific code in a function. This function can be defined outside the render function but must only be called within it. |
| 44 | + |
| 45 | +**Example** |
| 46 | + |
| 47 | +```ts |
| 48 | +import { defineInstrument } from '/runtime/v1/@opendatacapture/runtime-core'; |
| 49 | +import { z } from '/runtime/v1/[email protected]'; |
| 50 | + |
| 51 | +function createButton() { |
| 52 | + const button = document.createElement('button'); |
| 53 | + button.textContent = 'Submit Instrument'; |
| 54 | + document.body.appendChild(button); |
| 55 | + return button; |
| 56 | +} |
| 57 | + |
| 58 | +export default defineInstrument({ |
| 59 | + // ... |
| 60 | + kind: 'INTERACTIVE', |
| 61 | + content: { |
| 62 | + render(done) { |
| 63 | + const button = createButton(); |
| 64 | + button.addEventListener('click', () => { |
| 65 | + done({ message: 'Hello World' }); |
| 66 | + }); |
| 67 | + } |
| 68 | + } |
| 69 | + // ... |
| 70 | +}); |
| 71 | +``` |
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