feat: Object.isPlainFunction utility

object/index.js

@@ -32,6 +32,7 @@ module.exports = {
 	isFiniteNumber: require("./is-finite-number"),
 	isNumberValue: require("./is-number-value"),
 	isObject: require("./is-object"),
+	isPlainFunction: require("./is-plain-function"),
 	isPlainObject: require("./is-plain-object"),
 	isPromise: require("./is-promise"),
 	isValue: require("./is-value"),

object/is-plain-function.js

@@ -0,0 +1,11 @@
+"use strict";
+
+var isClassStr = RegExp.prototype.test.bind(/^\s*class[\s{/}]/)
+  , fnToString = Function.prototype.toString;
+
+module.exports = function (fn) {
+	if (typeof fn !== "function") return false;
+	if (typeof fn.call !== "function") return false;
+	if (typeof fn.apply !== "function") return false;
+	return !isClassStr(fnToString.call(fn));
+};

test/.eslintrc.json

@@ -1,13 +1,14 @@
 {
-	"globals": {
-		"Symbol": true
-	},
-	"rules": {
-		"consistent-this": "off",
-		"id-length": "off",
-		"no-empty-function": "off",
-		"no-new-wrappers": "off",
-		"no-prototype-builtins": "off",
-		"no-shadow": "off"
-	}
+  "globals": {
+    "Symbol": true
+  },
+  "rules": {
+    "consistent-this": "off",
+    "id-length": "off",
+    "no-empty-function": "off",
+    "no-eval": "off",
+    "no-new-wrappers": "off",
+    "no-prototype-builtins": "off",
+    "no-shadow": "off"
+  }
 }

test/object/is-plain-function.js

@@ -0,0 +1,42 @@
+"use strict";
+
+var setPrototypeOf = require("../../object/set-prototype-of");
+
+module.exports = function (t, a) {
+	a(t(function () {}), true, "Function");
+	a(t({}), false, "Object");
+	a(t(), false, "Undefined");
+	a(t(null), false, "Null");
+	if (setPrototypeOf) {
+		a(
+			t(Object.setPrototypeOf(function () {}, Object.prototype)),
+			false,
+			"Function with non-function prototype"
+		);
+	}
+	var arrowfn;
+	try {
+		arrowfn = eval("(() => {})");
+	} catch (e) {}
+	if (arrowfn) {
+		a(t(arrowfn), true, "Arrow function");
+	}
+
+	var classFn;
+	try {
+		classFn = eval("(class {})");
+	} catch (e) {}
+	if (classFn) {
+		a(t(classFn), false, "Class");
+	}
+
+	var commentedClassFn;
+	try {
+		// Follows issue reported to ljhard/is-callable project:
+		// https://github.com/ljharb/is-callable/issues/4
+		commentedClassFn = eval("(class/*kkk*/\n//blah\n Bar\n//blah\n {})");
+	} catch (e) {}
+	if (commentedClassFn) {
+		a(t(commentedClassFn, false, "Class"), false, "Class with comments");
+	}
+};