FOR

The FOR keyword can be to iterate over all elements of an array. The general syntax is:

FOR variableName IN expression

There is also a special variant for graph traversals:

FOR vertexVariableName, edgeVariableName, pathVariableName IN traversalExpression

For this special case see the graph traversals chapter.

For views, there is a special (optional) SEARCH keyword:

FOR variableName IN viewName SEARCH searchExpression

Details can be found in the views chapter.

For all other cases read on:

Each array element returned by expression is visited exactly once. It is required that expression returns an array in all cases. The empty array is allowed, too. The current array element is made available for further processing in the variable specified by variableName.

FOR u IN users
  RETURN u

This will iterate over all elements from the array users (note: this array consists of all documents from the collection named "users" in this case) and make the current array element available in variable u. u is not modified in this example but simply pushed into the result using the RETURN keyword.

Note: When iterating over collection-based arrays as shown here, the order of documents is undefined unless an explicit sort order is defined using a SORT statement.

The variable introduced by FOR is available until the scope the FOR is placed in is closed.

Another example that uses a statically declared array of values to iterate over:

FOR year IN [ 2011, 2012, 2013 ]
  RETURN { "year" : year, "isLeapYear" : year % 4 == 0 && (year % 100 != 0 || year % 400 == 0) }

Nesting of multiple FOR statements is allowed, too. When FOR statements are nested, a cross product of the array elements returned by the individual FOR statements will be created.

FOR u IN users
  FOR l IN locations
    RETURN { "user" : u, "location" : l }

In this example, there are two array iterations: an outer iteration over the array users plus an inner iteration over the array locations. The inner array is traversed as many times as there are elements in the outer array. For each iteration, the current values of users and locations are made available for further processing in the variable u and l.