jQuery isFunction() Method

What Is isFunction()?

jQuery.isFunction(value) (alias $.isFunction) returns true when value is callable as a function (since jQuery 1.2). Use it to guard optional callbacks before calling them.

$.isFunction(value)  → true | false

$.isFunction(function () {})  // true
$.isFunction("text")          // false
$.isFunction($)               // true  (jQuery factory)
$.isFunction($("div"))        // false (collection)

// Prefer in new code:
typeof x === "function"

How to Use isFunction()

Test first, then call — or migrate to native typeof.

1. Have a value that might be a callback (plugin option, handler, done).
2. Call $.isFunction(value) (legacy) or typeof value === "function" (preferred).
3. If true, invoke it safely.
4. If false, skip or use a fallback like $.noop.

Pseudocode

function isFunction(value):
    return typeof value === "function"

// Typical callback guard:
if isFunction(done):
    done(result)
else:
    // skip or use noop

Cheat Sheet

Keep these facts handy while checking callables.

Live Preview

Test common values with $.isFunction and native typeof.

value = function () {}
$.isFunction(value) → true
typeof value === "function" → true

jQuery Examples

Short, copy-ready snippets. Open DevTools Console, or use Try It Yourself for a live editor.

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

true
false

function finish(result, done) {
  if ($.isFunction(done)) {
    done(result);
  }
}

finish("ok", function (msg) {
  console.log(msg);  // "ok"
});
finish("ok", null);  // no call, no error

ok

var fn = function () {};
var text = "nope";

console.log($.isFunction(fn));             // true
console.log(typeof fn === "function");     // true

console.log($.isFunction(text));           // false
console.log(typeof text === "function");   // false

true
true
false
false

console.log($.isFunction($));          // true
console.log($.isFunction($("body")));  // false
console.log($.isFunction($.noop));     // true

true
false
true

Performance Notes

Function checks are cheap. Prefer native typeof for clarity and to avoid a removed jQuery API — not for micro-speed gains.

Key Takeaways