Question: I am trying to modify Javascripts Array type with a method which will push a value to an array only if its is not already present.

Here is my code:

```
// add a method conditionally
Array.prototype.method = function (name, func){
    if(!this.prototype[name]){
        this.prototype[name] = func;
        return this;
    }
};

// exclusive push
Array.method('xadd', function(value){
    if(this.indexOf(value) === -1){
        this.push(value)
    };
    return this;
});

```

However when I run the code the scratchpad in Firefox returns:

```
/*
Exception: TypeError: Array.method is not a function
@Scratchpad/3:19:1
*/

```

I want a vanilla way of doing this. Not a library as I am writing an open source library.

Answer: 
First I would run a check to see if the method is already on the array. Don't go overridding existing prototype methods. In addition, you're not adding `func` to the prototype - you're adding it to the instances you'll be creating.

```
if (!('method' in Array.prototype)) {
    Array.prototype.method = function (name, func) {
        if (!this[name]) this[name] = func;
    }
}

```

Then you need to actually create your array instance:

```
var arr = [1,2];

```

At which point you can use the method you created to add the function. Note in your question your check was incorrect:

```
arr.method('xadd', function (value) {
    if (this.indexOf(value) === -1) {
        this.push(value)
    };
});

arr.xadd(3); // [1,2,3]

```

[DEMO](http://jsfiddle.net/057911v7/)
Borrowing from Andy & Nihey I have arrived at the following solution which modifies the Array type making 'xadd' conditionally available to all instances of Array

```
if (!('xpush' in Array.prototype)) {
  Array.prototype.xpush = function(value){
    if(this.indexOf(value) === -1){
      this.push(value);
    };
    return this
  };
}

var a = [1,2,3];
console.log(a); // Array [ 1, 2, 3 ]
a.xadd(5);
console.log(a); // Array [ 1, 2, 3, 5 ]
a.xadd(3);
console.log(a); // Array [ 1, 2, 3, 5 ] '3' already present so not added

```

A better name would be xpush() as it's behaviour is a variant of push().