Implement map filter function

README.md

@@ -172,6 +172,18 @@ const template = 'http://example.com/{section|wrap(_, _)}{?ids|wrap([, ", ", ])}
 //--> http://example.com/_news_/?ids=["ent1", "ent2", "ent5"]
 ```
 
+### `map(from=to[, from=to[, ...]])`
+Will map the given value to a replacement according to the `from=to` pairs
+given as arguments to the `map` function. If no match is found, the value is
+not replaced.
+
+```
+const params = {type: 'entry'}
+const template = 'http://example.com/{type|map(article=articles, entry=entries)}'
+...
+//--> http://example.com/entries
+```
+
 ## Max length
 RFC 6570 specifies a 'prefix modifier', that limits the length of the value, by
 suffixing a parameter with a colon and the max number of characters. This is

lib/filters/index.js

@@ -4,5 +4,6 @@ module.exports = {
   upper: require('./upper'),
   lower: require('./lower'),
   wrap: require('./wrap'),
-  max: require('./max')
+  max: require('./max'),
+  map: require('./map')
 }

lib/filters/map-test.js

@@ -0,0 +1,45 @@
+import test from 'ava'
+
+import map from './map'
+
+test('should exist', (t) => {
+  t.is(typeof map, 'function')
+})
+
+test('should map value', (t) => {
+  const value = 'entry'
+  const args = [
+    'account=accounts',
+    'entry=entries'
+  ]
+
+  const ret = map(value, ...args)
+
+  t.is(ret, 'entries')
+})
+
+test('should return value when no match', (t) => {
+  const value = 'entry'
+  const args = ['account=accounts']
+
+  const ret = map(value, ...args)
+
+  t.is(ret, 'entry')
+})
+
+test('should return value when no mappings', (t) => {
+  const value = 'entry'
+
+  const ret = map(value)
+
+  t.is(ret, 'entry')
+})
+
+test('should not map null value', (t) => {
+  const value = null
+  const args = ['null=nope']
+
+  const ret = map(value, ...args)
+
+  t.is(ret, null)
+})

lib/filters/map.js

@@ -0,0 +1,16 @@
+function map (value, ...mappings) {
+  if (value === null || value === undefined) {
+    return value
+  }
+
+  const searchFor = `${value}=`
+  const mapping = mappings.find((mapping) => mapping.startsWith(searchFor))
+
+  if (mapping) {
+    return mapping.substr(searchFor.length)
+  }
+
+  return value
+}
+
+module.exports = map