hugo/content/functions/index-function.md

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title	linktitle	description	godocref	date	publishdate	lastmod	categories	tags	signature	workson	hugoversion	relatedfuncs	deprecated	aliases	needsexample
index	index	Looks up the index(es) or key(s) of the data structure passed into it.	https://golang.org/pkg/text/template/#hdr-Functions	2017-02-01	2017-02-01	2017-02-01	functions						false		true

index looks up the index(es) or key(s) of the data structure passed into it.

From the godocs:

Returns the result of indexing its first argument by the following arguments. Thus "index x 1 2 3" is, in Go syntax, x[1][2][3]. Each indexed item must be a map, slice, or array.

In Go templates, you can't access array, slice, or map elements directly the same way you would in Go. For example, $.Site.Data.authors[.Params.authorkey] isn't supported syntax.

Instead, you have to use index, a function that handles the lookup for you.