Add is.convert to convert a test to a function

convert.js

@@ -0,0 +1,87 @@
+'use strict'
+
+module.exports = convert
+
+function convert(test) {
+  if (typeof test === 'string') {
+    return typeFactory(test)
+  }
+
+  if (test === null || test === undefined) {
+    return ok
+  }
+
+  if (typeof test === 'object') {
+    return ('length' in test ? anyFactory : matchesFactory)(test)
+  }
+
+  if (typeof test === 'function') {
+    return test
+  }
+
+  throw new Error('Expected function, string, or object as test')
+}
+
+function convertAll(tests) {
+  var results = []
+  var length = tests.length
+  var index = -1
+
+  while (++index < length) {
+    results[index] = convert(tests[index])
+  }
+
+  return results
+}
+
+// Utility assert each property in `test` is represented in `node`, and each
+// values are strictly equal.
+function matchesFactory(test) {
+  return matches
+
+  function matches(node) {
+    var key
+
+    for (key in test) {
+      if (node[key] !== test[key]) {
+        return false
+      }
+    }
+
+    return true
+  }
+}
+
+function anyFactory(tests) {
+  var checks = convertAll(tests)
+  var length = checks.length
+
+  return matches
+
+  function matches() {
+    var index = -1
+
+    while (++index < length) {
+      if (checks[index].apply(this, arguments)) {
+        return true
+      }
+    }
+
+    return false
+  }
+}
+
+// Utility to convert a string into a function which checks a given node’s type
+// for said string.
+function typeFactory(test) {
+  return type
+
+  function type(node) {
+    return Boolean(node && node.type === test)
+  }
+}
+
+// Utility to return true.
+function ok() {
+  return true
+}

index.js

@@ -1,7 +1,11 @@
 'use strict'
 
+var convert = require('./convert')
+
 module.exports = is
 
+is.convert = convert
+
 // Assert if `test` passes for `node`.   When a `parent` node is known the
 // `index` of node.
 // eslint-disable-next-line max-params
@@ -31,87 +35,3 @@ function is(test, node, index, parent, context) {
 
   return Boolean(check.call(context, node, index, parent))
 }
-
-function convert(test) {
-  if (typeof test === 'string') {
-    return typeFactory(test)
-  }
-
-  if (test === null || test === undefined) {
-    return ok
-  }
-
-  if (typeof test === 'object') {
-    return ('length' in test ? anyFactory : matchesFactory)(test)
-  }
-
-  if (typeof test === 'function') {
-    return test
-  }
-
-  throw new Error('Expected function, string, or object as test')
-}
-
-function convertAll(tests) {
-  var results = []
-  var length = tests.length
-  var index = -1
-
-  while (++index < length) {
-    results[index] = convert(tests[index])
-  }
-
-  return results
-}
-
-// Utility assert each property in `test` is represented in `node`, and each
-// values are strictly equal.
-function matchesFactory(test) {
-  return matches
-
-  function matches(node) {
-    var key
-
-    for (key in test) {
-      if (node[key] !== test[key]) {
-        return false
-      }
-    }
-
-    return true
-  }
-}
-
-function anyFactory(tests) {
-  var checks = convertAll(tests)
-  var length = checks.length
-
-  return matches
-
-  function matches() {
-    var index = -1
-
-    while (++index < length) {
-      if (checks[index].apply(this, arguments)) {
-        return true
-      }
-    }
-
-    return false
-  }
-}
-
-// Utility to convert a string into a function which checks a given node’s type
-// for said string.
-function typeFactory(test) {
-  return type
-
-  function type(node) {
-    return Boolean(node && node.type === test)
-  }
-}
-
-// Utility to return true.
-function ok() {
-  return true
-}

package.json

@@ -20,7 +20,8 @@
     "Titus Wormer <tituswormer@gmail.com> (https://wooorm.com)"
   ],
   "files": [
-    "index.js"
+    "index.js",
+    "convert.js"
   ],
   "dependencies": {},
   "devDependencies": {

readme.md

@@ -83,6 +83,41 @@ with `type` set to a non-empty `string`).
 
 `boolean?` — Whether `node` matches.
 
+### `is.convert(test)`
+
+Create a test function from `test`, that can later be called with a `node`,
+`index`, and `parent`.
+Useful if you’re going to test many nodes, for example when creating a utility
+where something else passes an is-compatible test.
+
+Can also be accessed with `require('unist-util-is/convert')`.
+
+For example:
+
+```js
+var u = require('unist-builder')
+var convert = require('unist-util-is/convert')
+
+var test = convert('leaf')
+
+var tree = u('tree', [
+  u('node', [u('leaf', '1')]),
+  u('leaf', '2'),
+  u('node', [u('leaf', '3'), u('leaf', '4')]),
+  u('leaf', '5')
+])
+
+var leafs = tree.children.filter((child, index) => test(child, index, tree))
+
+console.log(leafs)
+```
+
+Yields:
+
+```js
+[({type: 'leaf', value: '2'}, {type: 'leaf', value: '5'})]
+```
+
 ## Related
 
 *   [`unist-util-find-after`](https://github.com/syntax-tree/unist-util-find-after)