Conditional Javascript Filter

You could use the ternary operator to decide whether or not to apply the filter:

currentOption === 'all' ? assets : assets.filter(asset => asset.type === currentOption)

The mapping to images, that you added to the end of your question, could be written like this:

(currentOption === 'all' ? assets : assets.filter(asset => asset.type === currentOption))
    .map( asset => <img src={asset.url} /> )

I would go with what you suggested, more or less:

assets.filter(asset => currentOption === "all" || asset.type === currentOption);

Keep in mind that filter() iterates over all of the items anyway.