Issue: webr.evalR() not capturing R output into result.obj

[manojkumaryejjala]

Hi All,

The webr.evalR() function is not capturing the text output of R code into the result.obj property of the returned promise. While the R code executes (as confirmed by output appearing in the webr-main.ts logs), result.obj consistently returns undefined. This prevents the retrieval and display of R output within the browser.

Environment:
Browser: Chrome 133.0.6943.142
Operating System: Windows 11, Version 24H2
WebR Version: https://webr.r-wasm.org/latest/webr.mjs

HTML:

<title>WebR Test</title> <script type="module" src="script.js"></script> <textarea id="code" rows="10" placeholder="Enter R code here..."></textarea>

Run
JavaScript:

import { WebR } from "https://webr.r-wasm.org/latest/webr.mjs";

let webr = null;

document.addEventListener('DOMContentLoaded', async () => {
const output = document.getElementById('output');
webr = new WebR();
await webr.init();

document.getElementById('runButton').addEventListener('click', async () => {
const code = document.getElementById('code').value;
try {
let result = await webr.evalR(code);
console.log("R result:", result);
console.log("Keys of result:", Object.keys(result));
console.log("result.obj:", result.obj);

    if (result && result.obj) {
        output.innerHTML = String(result.obj);
    } else {
        output.innerHTML = "No output";
    }
} catch (err) {
    console.error("R error:", err);
    output.innerHTML = `Error: ${err.message}`;
}

});
});
3. Steps to Reproduce:

Create an HTML file with the provided structure.
Create a JavaScript file with the provided code.
Include the webr.mjs file from the specified CDN.
Open the HTML file in the specified browser.
Enter R code in the textarea (e.g., 1 + 1, print("test"), rnorm(10,5,1)).
Click the "Run" button.
Observe the developer console and the output area.
4. Expected Behavior:

The output of the R code should be captured in the result.obj property and displayed in the output div.

Actual Behavior:
The R code executes (as evidenced by output appearing in the webr-main.ts logs, e.g., webr-main.ts:318 [1] 2).
The result.obj property consistently returns undefined.
The output area displays "No output" or an error message if an R error occurs.
6. Additional Information:

I have tried using capture.output() and print() within the R code, but the issue persists.
I have tested with different R commands, including simple arithmetic, print() statements, and numerical data generation (rnorm()), with consistent results.
The issue appears to be related to how WebR captures the output of webr.evalR().
The toArray() function also does not return useable data.
I have tried several different versions of my javascript file, and html file.
I have tried using different browsers.
I have tested on multiple computers.
7. Console Log Output:

(Paste the console log output here, including the R result, Keys of result, and result.obj logs.)

webr-main.ts:321 WebR is using PostMessage communication channel, nested R REPLs are not available.
webr-main.ts:318 [1] 2
script.js:14 R result: Proxy(Object) {obj: {…}, payloadType: 'ptr'}
script.js:15 Keys of result: (2) ['obj', 'payloadType']
script.js:16 result.obj: undefined
script.js:18 No output
8. Request:

Please investigate this issue and provide guidance on how to correctly capture R output using webr.evalR(). If this is a bug, please provide a fix or workaround.

Thank you for your time and assistance.

[georgestagg]

There seems to be some confusion here about how R evaluates statements and how the results are returned to webR. The webR.evalR() function captures the result of evaluated R code, not the console output, as you seem to be expecting.

Often R results are autoprinted, so that the resulting R objects are also printed as console output, but not always.

That is why your code is failing, you cannot simply use String() to turn an R object into text, because webR is actually returning a proxy to the resulting R object itself inside R's memory. The documentation explains this more here and here.

In any case, there is a simpler solution avoiding the need to work with webR proxies. Simply use the captureR() API to capture console output as an array of objects:

const webr = new WebR();
await webr.init();
const code = `
   1 + 2
   print("hello")
   rnorm(10)
`
const { output } = await webr.globalShelter.captureR(code, { withAutoprint: true });
console.log(output);

(4) [{…}, {…}, {…}, {…}]
0: {type: 'stdout', data: '[1] 3'}
1: {type: 'stdout', data: '[1] "hello"'}
2: {type: 'stdout', data: ' [1]  0.5503869 -1.4349522 -0.1838187 -1.7187045 -2.0486986  1.9307022'}
3: {type: 'stdout', data: ' [7] -0.5552701 -0.6423450  0.6618837 -0.2840043'}

More information about capturing R output is given in the documentation here.