diff --git a/function.js b/function.js
index 8bd8caf..3cac21e 100644
--- a/function.js
+++ b/function.js
@@ -151,3 +151,36 @@ export const equalityDeep = (a, b) => {
 // @ts-ignore
 export const isOneOf = (value, options) => options.includes(value)
 /* c8 ignore stop */
+
+export const isArray = array.isArray
+
+/**
+ * @param {any} s
+ * @return {s is String}
+ */
+export const isString = (s) => s && s.constructor === String
+
+/**
+ * @param {any} n
+ * @return {n is Number}
+ */
+export const isNumber = n => n != null && n.constructor === Number
+
+/**
+ * @template TYPE
+ * @param {any} n
+ * @param {TYPE} T
+ * @return {n is InstanceType<TYPE>}
+ */
+export const is = (n, T) => n && n.constructor === T
+
+/**
+ * @template TYPE
+ * @param {TYPE} T
+ */
+export const isTemplate = (T) =>
+  /**
+   * @param {any} n
+   * @return {n is InstanceType<TYPE>}
+   **/
+  n => n && n.constructor === T
diff --git a/function.test.js b/function.test.js
index 21b9b0b..dccd975 100644
--- a/function.test.js
+++ b/function.test.js
@@ -10,6 +10,45 @@ export const testBasics = _tc => {
   t.assert(calls === 1)
   t.assert(f.isOneOf(1, [3, 2, 1]))
   t.assert(!f.isOneOf(0, [3, 2, 1]))
+  // test is*
+  const arr = [1, 'two', [1], { one: 1 }]
+  arr.forEach(val => {
+    if (f.isArray(val)) {
+      /**
+       * @type {Array<any>}
+       */
+      const yy = val
+      t.assert(yy)
+    }
+    if (f.isString(val)) {
+      /**
+       * @type {string}
+       */
+      const yy = val
+      t.assert(yy)
+    }
+    if (f.isNumber(val)) {
+      /**
+       * @type {number}
+       */
+      const yy = val
+      t.assert(yy)
+    }
+    if (f.is(val, String)) {
+      /**
+       * @type {string}
+       */
+      const yy = val
+      t.assert(yy)
+    }
+    if (f.isTemplate(Number)(val)) {
+      /**
+       * @type {number}
+       */
+      const yy = val
+      t.assert(yy)
+    }
+  })
 }
 
 /**