How to prevent a method call, when that's not exist

Question

In my object init, I am calling the methods with their names. But some time, those methods may not declared or I don't want to call them. in case of that chance, how to prevent my method from call?

here is my calling approach : this[collectionName](); - here the name is parameter I receive. so the method is declared within object.

here is full code :

init: function( collectionName, size ){

            if( (typeof this[collectionName] ) === undefined ) return; //but not works!!!

            this.collectionName = collectionName;
            this.size =  size.toUpperCase() == "SMALL"  ? 20 : size.toUpperCase() == "MEDIUM" ? 35 : lsize.toUpperCase() == "LARGE" ? 50 : "SMALL";

            this[collectionName]();//some time method will not exist. how to check the existence and prevent it from call?
            return this.generateRecords();

        }

I am getting an error when the method is not their then :

New-DataModels.js?bust=1491457640410:69 Uncaught TypeError: this[collectionName] is not a function

You can check `this[collectionName]` exista and is a function — Satpal, Apr 06 '17 at 05:55
that's what I have written a condition, but not working for me. see my commet — 3gwebtrain, Apr 06 '17 at 05:55
better check for (typeof this[collectionName] === "function") — Vinod Louis, Apr 06 '17 at 05:55

Yeldar Kurmangaliyev · Answer 1 · 2017-04-06T07:04:27.143

A variables does exist and is declared, because it wouldn't enter function if it didn't exist because of this:

// it must be === "undefined" (in quotes) actually, not === undefined
if( (typeof this[collectionName] ) === "undefined" ) return;

However, as mentionted in error, the problem is that

this[collectionName] is not a function

i.e. this[collectionName] does exist, but it is not a function, and thereby you cannot call it.

You can change your function to make sure that this[collectionName] is a function:

init: function( collectionName, size ){
    if (typeof this[collectionName] !== 'function') return;

    this.collectionName = collectionName;
    this.size =  size.toUpperCase() == "SMALL"  ? 20 : size.toUpperCase() == "MEDIUM" ? 35 : lsize.toUpperCase() == "LARGE" ? 50 : "SMALL";

    this[collectionName]();//some time method will not exist. how to check the existence and prevent it from call?
    return this.generateRecords();
}

score 2 · Answer 2 · answered Apr 06 '17 at 05:56

You almost got it, just need a small modification in checking the typeof your property. typeof returns a string indicating the type of that object.

if( (typeof this[collectionName] ) === 'undefined' ) return;
// notice how I made 'undefined' into a string

Although I think it would be better if you check if its not a function:

if (typeof this[collectionName] !== 'function') return;

How to prevent a method call, when that's not exist

2 Answers2