mapbox API : how can I set a fallback for the 'get' expression?

I use this expression to set the line-color of a layer.

map.addLayer({
  ...
  'paint': {
    'line-color': ['get', 'color'],
  },
  ...
})

But what if the color attribute is not defined ? Is there a way to set a fallback value ? Thanks.

You can easily do this with the coalesce expression (docs here), which will always resolve to the first non-null value in the provided list of values.

In your example, this would something like:

map.addLayer({
  ...
  'paint': {
    'line-color': ['coalesce', ['get', 'color'], '#00ffff']
  },
  ...
})

There's a more involved example of this expression being used to determine an icon fallback here.

⚠️ disclaimer: I currently work at Mapbox ⚠️