Invoking a Function Without Parentheses

Invoking a function without parentheses

There are several different ways to call a function without parentheses.

Let's assume you have this function defined:

function greet() {
    console.log('hello');
}

Then here follow some ways to call greet without parentheses:

1. As Constructor

With new you can invoke a function without parentheses:

new greet; // parentheses are optional in this construct.

From MDN on the new oprator:

Syntax

new constructor[([arguments])]

2. As toString or valueOf Implementation

toString and valueOf are special methods: they get called implicitly when a conversion is necessary:

var obj = {
    toString: function() {
         return 'hello';
    }
}

'' + obj; // concatenation forces cast to string and call to toString.

You could (ab)use this pattern to call greet without parentheses:

'' + { toString: greet };

Or with valueOf:

+{ valueOf: greet };

valueOf and toString are in fact called from the @@toPrimitive method (since ES6), and so you can also implement that method:

+{ [Symbol.toPrimitive]: greet }
"" + { [Symbol.toPrimitive]: greet }

2.b Overriding valueOf in Function Prototype

You could take the previous idea to override the valueOf method on the Function prototype:

Function.prototype.valueOf = function() {
    this.call(this);
    // Optional improvement: avoid `NaN` issues when used in expressions.
    return 0; 
};

Once you have done that, you can write:

+greet;

And although there are parentheses involved down the line, the actual triggering invocation has no parentheses. See more about this in the blog "Calling methods in JavaScript, without really calling them"

3. As Generator

You could define a generator function (with *), which returns an iterator. You can call it using the spread syntax or with the for...of syntax.

First we need a generator variant of the original greet function:

function* greet_gen() {
    console.log('hello');
}

And then we call it without parentheses by defining the @@iterator method:

[...{ [Symbol.iterator]: greet_gen }];

Normally generators would have a yield keyword somewhere, but it is not needed for the function to get called.

The last statement invokes the function, but that could also be done with destructuring:

[,] = { [Symbol.iterator]: greet_gen };

or a for ... of construct, but it has parentheses of its own:

for ({} of { [Symbol.iterator]: greet_gen });

Note that you can do the above with the original greet function as well, but it will trigger an exception in the process, after greet has been executed (tested on FF and Chrome). You could manage the exception with a try...catch block.

4. As Getter

@jehna1 has a full answer on this, so give him credit. Here is a way to call a function parentheses-less on the global scope, avoiding the deprecated __defineGetter__ method. It uses Object.defineProperty instead.

We need to create a variant of the original greet function for this:

Object.defineProperty(window, 'greet_get', { get: greet });

And then:

greet_get;

Replace window with whatever your global object is.

You could call the original greet function without leaving a trace on the global object like this:

Object.defineProperty({}, 'greet', { get: greet }).greet;

But one could argue we do have parentheses here (although they are not involved in the actual invocation).

5. As Tag Function

Since ES6 you can call a function passing it a template literal with this syntax:

greet``;

See "Tagged Template Literals".

6. As Proxy Handler

Since ES6, you can define a proxy:

var proxy = new Proxy({}, { get: greet } );

And then reading any property value will invoke greet:

proxy._; // even if property not defined, it still triggers greet

There are many variations of this. One more example:

var proxy = new Proxy({}, { has: greet } );

1 in proxy; // triggers greet

7. As instance checker

The instanceof operator executes the @@hasInstance method on the second operand, when defined:

1 instanceof { [Symbol.hasInstance]: greet } // triggers greet

Difference of calling a function with and without parentheses in JavaScript

With parentheses the method is invoked because of the parentheses, and the result of that invocation will be stored in before_add.

Without the parentheses you store a reference (or "pointer" if you will) to the function in the variable. That way it will be invoked whenever someone invokes before_add().

If that didn't clear things up, maybe this will help:

function Foo() {    return 'Cool!';}
function Bar(arg) {    console.log(arg);}
// Store the >>result of the invocation of the Foo function<< into Xvar x = Foo();console.log(x);
// Store >>a reference to the Bar function<< in yvar y = Bar;// Invoke the referenced methody('Woah!');
// Also, show what y is:console.log(y);
// Now, try Bar **with** parentheses:var z = Bar('Whut?');
// By now, 'Whut?' as already been output to the console; the below line will// return undefined because the invocation of Bar() didn't return anything.console.log(z);

Javascript function call with/without parentheses

setTimeout(foo, 2000) passes the function foo and the number 2000 as arguments to setTimeout. setTimeout(foo(), 2000) calls foo and passes its return value and the number 2000 to setTimeout.

In the first example, you’re not calling the function at all, just passing it as an argument like any other value.

As a simpler example, just log it:

function foo() {
    return 5;
}

console.log(foo);   // console.log is passed a function and prints [Function]

console.log(foo()); // foo() is called and returns 5; console.log is passed 5
                    // and prints 5

call function without parenthesis

the jquery identifier $ can be called without its parenthesis.

No. The function isn't being called.

Functions are objects. Objects can have properties. This is simply accessing a property of the function object.

function foo() {  return 1;}
foo.bar = 2;
alert(foo.bar);alert(foo());

What is the difference between calling the function without parentheses and with parentheses

_incrementCounter inside onPressed is a function reference, which basically means it is not executed immediately, it is executed after the user clicks on the specific widget.(callback)

_incrementCounter() is a function call and it is executed immediately.

Therefore, inside onPressed you can either pass a function reference or an anonymous function that will act as a callback.

floatingActionButton: FloatingActionButton(
    onPressed: _incrementCounter,
    tooltip: 'Increment',
    child: Icon(Icons.add),
  ),

or

floatingActionButton: FloatingActionButton(
    onPressed: () {
        // Add your onPressed code here!
      },
    tooltip: 'Increment',
    child: Icon(Icons.add),
  ),

The is not something specific to dart, it is also done in javascript and many other languages:

What is the difference between a function call and function reference?

Javascript function call with/without parentheses

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