if (typeof v === 'function') {
    // do something
}

Sure underscore’s way is more efficient, but the best way to check, when efficiency isn’t an issue, is written on underscore’s page linked by @Paul Rosania.

Inspired by underscore, the final isFunction function is as follows:

function isFunction(functionToCheck) {
 return functionToCheck && {}.toString.call(functionToCheck) === '[object Function]';
}

Note: This solution doesn’t work for async functions, generators or proxied functions. Please see other answers for more up to date solutions.

There are several ways so I will summarize them all

1. Best way is:

   function foo(v) {if (v instanceof Function) {/* do something */} };
   

   Most performant (no string comparison) and elegant solution – the instanceof operator has been supported in browsers for a very long time, so don’t worry – it will work in IE 6.

2. Next best way is:

   function foo(v) {if (typeof v === "function") {/* do something */} };
   

   disadvantage of typeof is that it is susceptible to silent failure, bad, so if you have a typo (e.g. “finction”) – in this case the if will just return false and you won’t know you have an error until later in your code

3. The next best way is:

   function isFunction(functionToCheck) {
       var getType = {};
       return functionToCheck && getType.toString.call(functionToCheck) === '[object Function]';
   }
   

   This has no advantage over solution #1 or #2 but is a lot less readable. An improved version of this is

   function isFunction(x) {
       return Object.prototype.toString.call(x) == '[object Function]';
   }
   

   but still lot less semantic than solution #1

Underscore.js uses a more elaborate but highly performant test:

_.isFunction = function(obj) {
  return !!(obj && obj.constructor && obj.call && obj.apply);
};

See: https://jsben.ch/B6h73

EDIT: updated tests suggest that typeof might be faster, see https://jsben.ch/B6h73

jQuery (deprecated since version 3.3) Reference

$.isFunction(functionName);

AngularJS Reference

angular.isFunction(value);

Lodash Reference

_.isFunction(value);

Underscore Reference

_.isFunction(object); 

Node.js deprecated since v4.0.0 Reference

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

@grandecomplex: There’s a fair amount of verbosity to your solution. It would be much clearer if written like this:

function isFunction(x) {
  return Object.prototype.toString.call(x) == '[object Function]';
}

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) : false;

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

Try the instanceof operator: it seems that all functions inherit from the Function class:

// Test data
var f1 = function () { alert("test"); }
var o1 = { Name: "Object_1" };
F_est = function () { };
var o2 = new F_est();

// Results
alert(f1 instanceof Function); // true
alert(o1 instanceof Function); // false
alert(o2 instanceof Function); // false

An other simply way:

var fn = function () {}
if (fn.constructor === Function) {
  // true
} else {
  // false
}

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.

If you use Lodash you can do it with _.isFunction.

_.isFunction(function(){});
// => true

_.isFunction(/abc/);
// => false

_.isFunction(true);
// => false

_.isFunction(null);
// => false

This method returns true if value is a function, else false.

This is an old question but there are some considerations in 2022:

First, browser compatibility: instanceof is supported by all modern browsers as well as Deno and NodeJS.
Also, it’s syntactically readable and more friendly than typeof.
Finally, it provides a good performance over string comparison but is slower than typeof. Therefore, for me this is the a good option

const fnc = () => {}
const isFunction = f => !!f && f instanceof Function
const isFunctionFaster = f => !!f && 'function' === typeof f

console.log({
  isFunction: isFunction(fnc),
  isFunctionFaster: isFunctionFaster(fnc),
})

Notice

It is important to understand that this is a optimized function for benchmarking. When you bench mark you want to pass all the test like null, undefined and some on possible parameters received. f && ... filter this null like parameters to reduce computation time.

• Caveats of instanceof operator:

This operator tests the presence of constructor.prototype in the object’s prototype chain. This usually (though not always) means object was constructed with constructor. Therefore, this process is slower compared with typeof operator.

typeof v === ‘function’)

• Caveats of typeof operator:

This operator returns a string indicating the type of the operand’s value. This is executed very fast.

• Caveats of instanceof and typeof operators:

Remember that a class declaration, it’s also considered as a function by the these operators, as you can see in this snippet:

// Class Declaration
class A {}

// Instances
const obj = {}
const arr = []
const fnc = () => {}
const a = new A()

console.log('typeof')
console.log(`Object[${typeof Object}], obj[${typeof obj}]`)
console.log(`Array[${typeof Array}], arr[${typeof arr}]`)
console.log(`Function[${typeof Function}], fnc[${typeof fnc}]`)
console.log(`A[${typeof A}], a[${typeof a}]`)

console.log('instanceof')
console.log(`Object[${Object instanceof Object}], obj[${obj instanceof Object}]`)
console.log(`Array[${Array instanceof Array}], arr[${arr instanceof Array}]`)
console.log(`Function[${Function instanceof Function}], fnc[${fnc instanceof Function}]`)
console.log(`A[${A instanceof A}], a[${a instanceof A}]`)

Here is a basic sample of the isFunction and isFunctionFaster usage with different instances:

// Functions
const isNil = o => o == null
const isFunction = f => !!f && f instanceof Function
const isFunctionFaster = f => !!f && 'function' === typeof f

class A {}

function basicFnc(){}
async function asyncFnc(){}

const arrowFnc = ()=> {}
const arrowRFnc = ()=> 1

// Not functions
const obj = {}
const arr = []
const str = 'function'
const bol = true
const num = 1
const a = new A()

const list = [
    isFunction,
    isFunctionFaster,
    basicFnc,
    arrowFnc,
    arrowRFnc,
    asyncFnc,
    Array,
    Date,
    Object,
    Number,
    String,
    Symbol,
    A,
    obj,
    arr,
    str,
    bol,
    num,
    a,
    null,
    undefined,
]

for (const arg of list) {
  console.log(`${arg} is a function: ${isFunction(arg)}`)
}

Here is a basic benchmark of these functions:

/**
 * Figure out how long it takes for a method to execute.
 * 
 * @param {Function} method to test 
 * @param {number} iterations number of executions.
 * @param {Array} args to pass in. 
 * @param {T} context the context to call the method in.
 * @return {number} the time it took, in milliseconds to execute.
 */
const bench = (method, list, iterations, context) => {
    let start = 0
    const timer = action => {
        const time = performance.now()
        switch (action) {
            case 'start':
                start = time
                return 0
            case 'stop':
                const elapsed = time - start
                start = 0
                return elapsed
            default:
                return time - start
        }
    };

    const result = []
    timer('start')
    list = [...list]
    for (let i = 0; i < iterations; i++) {
      for (const args of list) {
        result.push(method.apply(context, args))
      }
    }
    const elapsed = timer('stop')
    
    console.log(`Called method [${method.name}]`)
    console.log(`Mean: ${elapsed / iterations}`)
    console.log(`Exec. time: ${elapsed}`)

    return elapsed
}

const fnc = () => {}
const isFunction = (f) => f && f instanceof Function
const isFunctionFaster = (f) => f && 'function' === typeof f


class A {}

function basicFnc(){}
async function asyncFnc(){}

const arrowFnc = ()=> {}
const arrowRFnc = ()=> 1

// Not functions
const obj = {}
const arr = []
const str = 'function'
const bol = true
const num = 1
const a = new A()

const list = [
    [isFunction],
    [basicFnc],
    [arrowFnc],
    [arrowRFnc],
    [asyncFnc],
    [Array],
    [Date],
    [Object],
    [Number],
    [String],
    [Symbol],
    [A],
    [obj],
    [arr],
    [str],
    [bol],
    [num],
    [a],
    [null],
    [undefined],
]

const e1 = bench(isFunction, list, 10000)
const e2 = bench(isFunctionFaster, list, 10000)

const rate = e2/e1
const percent = Math.abs(1 - rate)*100

console.log(`[isFunctionFaster] is ${(percent).toFixed(2)}% ${rate < 1 ? 'faster' : 'slower'} than [isFunction]`)

Conclusion

In general isFunctionFaster is faster than isFunction in 30%.

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

I found that when testing native browser functions in IE8, using toString, instanceof, and typeof did not work. Here is a method that works fine in IE8 (as far as I know):

function isFn(f){
    return !!(f && f.call && f.apply);
}
//Returns true in IE7/8
isFn(document.getElementById);

Alternatively, you can check for native functions using:

"getElementById" in document

Though, I have read somewhere that this will not always work in IE7 and below.

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

you should use typeOf operator in js.

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

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

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

if you are looking for a simple solution:

 function isFunction(value) {
   return value instanceof Function
}

Becareful about this :

typeof Object === "function" // true.
typeof Array  === "function" // true

There are functions whose arguments can be passed either as a value or though a callback. To implement them a nice helper function that utilizes type detection is the following:

const toCallback = (v)=>(v instanceof Function)?v:()=>v

Then let’s say you want to implement a function that can receive either a fixed value or a callback that returns the value. See below:

let someFunction=(valueOrFunction)=>{
    let f = toCallback(valueOrFunction); 
    [1,2,3].forEach(v=>console.log(`value passed: ${f()}`))   
}

and now you can execute the above in two ways:

someFunction(5);
//value passed: 5
//value passed: 5
//value passed: 5

someFunction(()=>Math.random());
//value passed: 0.3218832537523002
//value passed: 0.2343617971814611
//value passed: 0.38216627030533656

This is a bit tricky question as you need to define “what is a function?” first.
Let’s look at node (V8 implementation of JavaScript engine). It is written in C++ but also has parts in JavaScript.
Some functions are magically mapped to C++ functions. Those do not have all the properties of standard function.
Other functions are simply JavaScript functions with prototype set to null. It means they don’t inherit from Object and wouldn’t have methods such as toString() and similar.
Then finally you have standard instances of Function.
And there is also AsyncFunction which is a Function but a bit different.
Think what about a promise that returns a function – is it a Function or rather a Promise? Is Promise a Function?

This told let’s see how node implements checking if an argument is an AsyncFunction:
readable-stream defined AsyncFunction as a prototype of async function.
Then type check is simply typeof arg === 'function' as seen here.

• You need to define “what is a function” first
• If you understand it as an instance of some class then you can use for example arg instanceof Function which returns a boolean
• If you wish to see what the function inherits from then you could check its prototype
• If you define anything that can be called a function – check if it is callable
• There are still some issues with that 🙁

// It doesn't really matter if you use named function or not - but that is cool to know that it can have a name :)
let f = function aNamedFunction(){console.log('hello')};
f.prototype = null;
console.log(f instanceof Function, typeof f); // true, function
f.__proto__ = null;
console.log(f instanceof Function); // false, function
console.log(Object.getOwnPropertyNames(f)); // prototype, length, name
f(); // 'hello'

To summarize:
The typeof arg === 'function' seems pretty good (though there are few fancy scenarios where it would fail).