collections.IsSet

For example, consider this site configuration:

[params]
showHeroImage = false

It the value of showHeroImage is true, we can detect that it exists using either if or with:

{{ if site.Params.showHeroImage }}
  {{ site.Params.showHeroImage }} → true
{{ end }}

{{ with site.Params.showHeroImage }}
  {{ . }} → true
{{ end }}

But if the value of showHeroImage is false, we can't use either if or with to detect its existence. In this case, you must use the isset function:

{{ if isset site.Params "showheroimage" }}
  The showHeroImage parameter is set to {{ site.Params.showHeroImage }}.
{{ end }}

When using the isset function you must reference the key using lower case. See the previous example.