Check if a variable is of function type

Question

Suppose I have any variable, which is defined as follows:

var a = function() {/* Statements */};

I want a function which checks if the type

Answer 1

Something with more browser support and also include async functions could be:

const isFunction = value => value && (Object.prototype.toString.call(value) === "[object Function]" || "function" === typeof value || value instanceof Function);

and then test it like:

isFunction(isFunction); //true
isFunction(function(){}); //true
isFunction(()=> {}); //true
isFunction(()=> {return 1}); //true
isFunction(async function asyncFunction(){}); //true
isFunction(Array); //true
isFunction(Date); //true
isFunction(Object); //true
isFunction(Number); //true
isFunction(String); //true
isFunction(Symbol); //true
isFunction({}); //false
isFunction([]); //false
isFunction("function"); //false
isFunction(true); //false
isFunction(1); //false
isFunction("Alireza Dezfoolian"); //false

Answer 2

For those who's interested in functional style, or looks for more expressive approach to utilize in meta programming (such as type checking), it could be interesting to see Ramda library to accomplish such task.

Next code contains only pure and pointfree functions:

const R = require('ramda');

const isPrototypeEquals = R.pipe(Object.getPrototypeOf, R.equals);

const equalsSyncFunction = isPrototypeEquals(() => {});

const isSyncFunction = R.pipe(Object.getPrototypeOf, equalsSyncFunction);

As of ES2017, async functions are available, so we can check against them as well:

const equalsAsyncFunction = isPrototypeEquals(async () => {});

const isAsyncFunction = R.pipe(Object.getPrototypeOf, equalsAsyncFunction);

And then combine them together:

const isFunction = R.either(isSyncFunction, isAsyncFunction);

Of course, function should be protected against null and undefined values, so to make it "safe":

const safeIsFunction = R.unless(R.isNil, isFunction);

And, complete snippet to sum up:

const R = require('ramda');

const isPrototypeEquals = R.pipe(Object.getPrototypeOf, R.equals);

const equalsSyncFunction = isPrototypeEquals(() => {});
const equalsAsyncFunction = isPrototypeEquals(async () => {});

const isSyncFunction = R.pipe(Object.getPrototypeOf, equalsSyncFunction);
const isAsyncFunction = R.pipe(Object.getPrototypeOf, equalsAsyncFunction);

const isFunction = R.either(isSyncFunction, isAsyncFunction);

const safeIsFunction = R.unless(R.isNil, isFunction);

// ---

console.log(safeIsFunction( function () {} ));
console.log(safeIsFunction( () => {} ));
console.log(safeIsFunction( (async () => {}) ));
console.log(safeIsFunction( new class {} ));
console.log(safeIsFunction( {} ));
console.log(safeIsFunction( [] ));
console.log(safeIsFunction( 'a' ));
console.log(safeIsFunction( 1 ));
console.log(safeIsFunction( null ));
console.log(safeIsFunction( undefined ));

However, note the this solution could show less performance than other available options due to extensive usage of higher-order functions.

Answer 3

The below seems to work for me as well (tested from node.js):

var isFunction = function(o) {
     return Function.prototype.isPrototypeOf(o);
};

console.log(isFunction(function(){})); // true
console.log(isFunction({})); // false

Answer 4

you should use typeOf operator in js.

var a=function(){
    alert("fun a");
}
alert(typeof a);// alerts "function"

Answer 5

Since node v0.11 you can use the standard util function :

var util = require('util');
util.isFunction('foo');

Answer 6

I think you can just define a flag on the Function prototype and check if the instance you want to test inherited that

define a flag:

Function.prototype.isFunction = true; 

and then check if it exist

var foo = function(){};
foo.isFunction; // will return true

The downside is that another prototype can define the same flag and then it's worthless, but if you have full control over the included modules it is the easiest way