Opencv.js How to apply a per pixel operation efficiently?

Assume I have a 3 channel image to which I want to do some transformation.

For example, say I want to make all white pixels red.

In python I could do something like:

img[img == (255, 255, 255)] = (255, 0, 0)

Is there any equivalent syntax in Javascript to be applied to OpenCV.js matrices?

Iterating through each pixel in the image manually is terribly slow.