Google earth engine - set null band values as zero

Question

I have a function which assigns a score depend on the value of a band, which I then map to an image collection.

However, in some cases, I have null values, where no value is given. In these cases I'd like to convert the null values to 0. I tried to set null values as zero in the code, but I'm getting an image error. My new code, with the extra line is below.

function currentHS_function(image){
  var currentHS = image.expression(
 "(b('current') < 0.13) ? 1.0" +
 ": (b('current') > 0.15) ? 0.0" +
 ": (b('current') = null) ? 0.0" + // new line - set null values as 0
 ": -50.0*current + 15.0/2.0 ",
 {"current" : image});
   return currentHS;
 }

// apply heath score function to image collection
var currentHS_collection = currentSpeed.map(currentHS_function);

Answer 1

One thing you can do is to unmask the image using a proxy value before the expression and then catch that proxy value:

function currentHS_function(image){
  var proxy = -999
  image = image.unmask(proxy)
  var currentHS = image.expression(
 "(b('current') == proxy) ? 0.0" +
 ": (b('current') < 0.13) ? 1.0" +
 ": (b('current') > 0.15) ? 0.0" +
 ": -50.0*current + 15.0/2.0 ",
 {"current" : image, "proxy": proxy});
   return currentHS;
 }

In this case -999 is the proxy value, but you can use one of your own