This gist reproduces the cafee (which is default) image preprocessing used in keras.applications (up to a rescaling factor) with tensorflow.js.

Keras applications cafee image preprocessing.js

let img = await webcam.capture();

let rChannel = img.slice([0,0,0],[192,192,1]).reshape([192,192]);
let gChannel = img.slice([0,0,1],[192,192,1]).reshape([192,192]);
let bChannel = img.slice([0,0,2],[192,192,1]).reshape([192,192]);

img = tf.stack([bChannel, gChannel, rChannel], 2);

img = tf.mul(img, 1/255);
img = tf.add(img, -.5);
img = tf.add(img, [0.0923, 0.0420, 0.1498]);