Details and Options

Scan[f,expr] discards the results of applying f to the subexpressions in expr. Unlike Map, Scan does not build up a new expression to return.
You can use Return to exit from Scan. Return[ret] causes the final value of Scan to be ret. If no explicit return values are specified, the final result from Scan is Null.
You can also use Throw to exit from Scan. »
Scan is useful in carrying out an operation on parts of expressions where the operation has a "side effect", such as making an assignment.
Scan uses standard level specifications:
n levels 1 through n

Infinity levels 1 through Infinity

{n} level n only

{n₁,n₂} levels n₁ through n₂
The default value for levelspec in Scan is {1}.
A positive level n consists of all parts of expr specified by n indices.
A negative level -n consists of all parts of expr with depth n.
Level -1 consists of numbers, symbols, and other objects that do not have subparts.
Level 0 corresponds to the whole expression.
With the option setting Heads->True, Scan includes heads of expressions and their parts. »
Scan traverses the parts of expr in a depth-first order, with leaves visited before roots.
If expr is an Association object, Scan[f,expr] applies f only to the values in the association.
If expr is a SparseArray object, Scan[f,expr] applies f only to the values or subarrays that explicitly appear in expr.
Scan[f][expr] is equivalent to Scan[f,expr].
Parallelize[Scan[f,expr]] computes Scan[f,expr] in parallel on all subkernels. »