NodeJS Sharp Provides Incorrect Quality For JPEG Images - node.js

Question

I am working with sharp node package to calculate the quality of an image.
I couldn't find any API in the package which would calculate the quality.
So I came up with an implementation that follows following steps -
Accept incoming image buffer & create sharp instance
Create another instance from this instance by setting quality to 100
Compare the size of original sharp instance and new sharp instance
If there is a difference in the size, update the quality and execute step 2 with updated quality
Return the quality once comparison in step 4 gives smallest difference
I tested this approach by using an image of known quality i.e. 50 (confirmed)
EDIT - I generated images with different quality values using Photoshop
However, the above logic returns the quality as 82 (expected is something close to 50)
Problem
So the problem is I am not able to figure out the quality of image.
It is fine if the above logic returns a closest value such as 49 or 51.
However the result is totally different.
Results
As per this logic, I get following results for a given quality -
Actual Quality 50 - Result 82
Actual Quality 60 - Result 90
Actual Quality 70 - Result 90
Actual Quality 80 - Result 94
Actual Quality 90 - Result 98
Actual Quality 100 - Result 98
Code
The following code snippet is used to calculate the quality.
I do understand that it is needs improvements for precise results.
But it should at least provide close values.
async function getJpegQuality(image: sharp.Sharp, min_quality: number, max_quality: number): Promise<number> {
if (Math.abs(max_quality - min_quality) <= 1) {
return max_quality;
}
const updated_image: sharp.Sharp = sharp(await image.jpeg({ quality: max_quality }).toBuffer());
const [metadata, updated_metadata]: sharp.Metadata[] = await Promise.all([image.metadata(), updated_image.metadata()]);
// update quality as per size comparison
if (metadata.size > updated_metadata.size) {
const temp_max = Math.round(max_quality);
max_quality = Math.round((max_quality * 2) - min_quality);
min_quality = Math.round(temp_max);
} else {
max_quality = Math.round((min_quality + max_quality) / 2);
min_quality = Math.round((min_quality + max_quality) / 2);
}
// recursion
return await getJpegQuality(image, min_quality, max_quality);
}
Usage
const image: sharp.Sharp = sharp(file.originalImage.buffer);
const quality = await getJpegQuality(image, 1, 100);
console.log(quality);
Thanks!