Clojure 1.11 added a function for this to clojure.core, the standard library. Clojure 1.11 was released on 2022-03-22.

update-vals

(update-vals m f) applies the function to every value in the map. It returns a new map {k (f v) ...}.

Usage

(update-vals {:a 1 :b 2} str)
;;        => {:a "1", :b "2"}

See also: update-keys

(update-keys m f) applies the function to every key in the map. It returns a new map {(f k) v ...}. All keys resulting from (f k) must be unique.

This function, too, was added in Clojure 1.11.

Usage

(update-keys {:a 1 :b 2} str)
;;        => {":a" 1, ":b" 2}

map-map, map-map-keys, and map-map-values

I know of no existing function in Clojure for this, but here’s an implementation of that function as map-map-values that you are free to copy. It comes with two closely related functions, map-map and map-map-keys, which are also missing from the standard library:

(defn map-map
    "Returns a new map with each key-value pair in `m` transformed by `f`. `f` takes the arguments `[key value]` and should return a value castable to a map entry, such as `{transformed-key transformed-value}`."
    [f m]
    (into (empty m) (map #(apply f %) m)) )

(defn map-map-keys [f m]
    (map-map (fn [key value] {(f key) value}) m) )

(defn map-map-values [f m]
    (map-map (fn [key value] {key (f value)}) m) )

Usage

You can call map-map-values like this:

(map-map-values str {:a 1 :b 2})
;;           => {:a "1", :b "2"}

And the other two functions like this:

(map-map-keys str {:a 1 :b 2})
;;         => {":a" 1, ":b" 2}
(map-map (fn [k v] {v k}) {:a 1 :b 2})
;;    => {1 :a, 2 :b}

Alternative implementations

If you only want map-map-keys or map-map-values, without the more general map-map function, you can use these implementations, which don’t rely on map-map:

(defn map-map-keys [f m]
    (into (empty m)
        (for [[key value] m]
            {(f key) value} )))

(defn map-map-values [f m]
    (into (empty m)
        (for [[key value] m]
            {key (f value)} )))

Also, here’s an alternative implementation of map-map that is based on clojure.walk/walk instead of into, if you prefer this phrasing:

(defn map-map [f m]
    (clojure.walk/walk #(apply f %) identity m) )

Parellel versions – pmap-map, etc.

There are also parallel versions of these functions if you need them. They simply use pmap instead of map.

(defn pmap-map [f m]
    (into (empty m) (pmap #(apply f %) m)) )
(defn pmap-map-keys [f m]
    (pmap-map (fn [key value] {(f key) value}) m) )
(defn pmap-map-values [f m]
    (pmap-map (fn [key value] {key (f value)}) m) )

(defn map-vals
  "Map f over every value of m.
   Returns a map with the same keys as m, where each of its values is now the result of applying f to them one by one.
   f is a function of one arg, which will be called which each value of m, and should return the new value.
   Faster then map-vals-transient on small maps (8 elements and under)"
  [f m]
  (reduce-kv (fn [m k v]
               (assoc m k (f v)))
             {} m))

(defn map-vals-transient
  "Map f over every value of m.
   Returns a map with the same keys as m, where each of its values is now the result of applying f to them one by one.
   f is a function of one arg, which will be called which each value of m, and should return the new value.
   Faster then map-vals on big maps (9 elements or more)"
  [f m]
  (persistent! (reduce-kv (fn [m k v]
                            (assoc! m k (f v)))
                          (transient {}) m)))

I'm wondering why nobody has mentioned the specter library yet. It has been written to make this kind of transform easy to code (and, even more important, the written code easy to understand), while still being very performant:

(require '[com.rpl.specter :as specter])

(defn map-vals [m f]
  (specter/transform
   [specter/ALL specter/LAST]
   f m))

(map-vals {:a "test" :b "testing"}
          #(.toUpperCase %))

Writing such a function in pure Clojure is simple, but the code gets much trickier once you move on to highly nested code composed of different data structures. And this is where specter comes in.

I recommend watching this episode on Clojure TV which explains the motivation behind and details of specter.

Clojure 1.7 (released June 30, 2015) provides an elegant solution for this with update:

(defn map-function-on-map-vals [m f]
    (->> (map #(update % 1 f) m)
         (into {})))

(map-function-on-map-vals {:a "test" :b "testing"} #(.toUpperCase %))
;; => {:a "TEST", :b "TESTING"}