Can i call constructor function in javascript?

Question

i have created a constructor function in javascript

function hello () {
    name = 'shahin';
    age= 22;
    mesg = function () {
        return 'My name is ' + this.name + 'and i am '+ this.age + 'old';
    }
}

console.log(hello.mesg());

and instead creating of new constructor from it i just wanted to whether it is working as a normal function or not. hence try with the console and see that error : "TypeError: hello.mesg is not a function.

`

function hello () {
    this.name = 'shahin';
    this.age= 22;
    this.mesg = function () {
        return 'My name is ' + this.name + ' and i am '+ this.age + ' years old';
    }
}

console.log(hello.mesg())

I even try with this and got the same error

If you call a constructor function without the new keyword, this will be bound to window. — evolutionxbox
– evolutionxbox, Commented Nov 27, 2017 at 13:55

moon · Accepted Answer · 2017-11-27 14:05:26Z

1

To understand your question properly, you need to know the return value of a function.

(function(){
    var a = 1;
})();

This returns undefined, since there's no any return value designated.

(function(){
    var b = 2;    
    return b;
})();

This obviously returns b.

Can you distinguish the difference? So, the case #1 and case #2, hello function doesn't point anything as a return value, that's why it returns undefined and you couldn't be able to access mesg method.

To work this out properly, there're bunch of possible ways, I'll give you one of examples.

function hello() {
    var name = 'shahin';
    var age= 22;
    var mesg = function () {
        return 'My name is ' + name + 'and i am '+ age + ' old';
    };

    return {
        getName: mesg
    };
}

var func = hello();
func.getName(); // print 'My name is ... '

edited Nov 27, 2017 at 14:05

answered Nov 27, 2017 at 13:56

moon

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2 Comments

evolutionxbox Over a year ago

getName will print "My name is undefinedand i am undefinedold" - Remove all this references and use lets/vars

evolutionxbox Over a year ago

you still need to remove this, otherwise you're adding the mesg function to window

gurvinder372 · Accepted Answer · 2017-11-27 13:48:15Z

0

In the second case, mesg is the instance's variable of hello, you need to invoke it by instantiating with new operator.

console.log((new hello()).mesg())

In the first case, you can't invoke mesg as a property. You invoke mesg only if you return the mesg function

function hello () {
    name = 'shahin';
    age= 22;
    return mesg = function () {
        return 'My name is ' + this.name + 'and i am '+ this.age + 'old';
    }
}
console.log(hello()());

edited Nov 27, 2017 at 13:48

answered Nov 27, 2017 at 13:43

gurvinder372

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6 Comments

charlietfl Over a year ago

Not in the first case it's not

Pointy Over a year ago

It's not a property of the prototype in either version.

ibrahim mahrir Over a year ago

The way OP is using hello, just calling mesg directly will work (in both cases).

evolutionxbox Over a year ago

where's the second function?

Sanjida lina Over a year ago

thank you @gurvinder372 what if there is two function inside this

function hello () {     name = 'shahin';     age= 22;     return mesg = function () {         return 'My name is ' + this.name + 'and i am '+ this.age + 'old';     }     return mesg1 = function () {         return 'My name is ' + this.name + 'and i am '+ this.age + 'old';     } }

how can i evoke the 2nd function there? i mean mesg1()

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