How to modify Array.prototype in Javascript

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.

method is a method of the Array.prototype object. Instances of the Array object and the prototype have that method. — Ram
– Ram, Commented Jun 20, 2015 at 18:57

Nihey Takizawa · Accepted Answer · 2015-06-20 19:01:41Z

3

When you're putting a method on Array.prototype the method will be available on the instances of Array.

// Add the custom method
Array.prototype.method = function() {
    console.log('XXX');
}

var foo = [];
// prints XXX
foo.method();

answered Jun 20, 2015 at 19:01

Nihey Takizawa

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Comments

Andy · Accepted Answer · 2015-06-20 19:57:31Z

2

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

edited Jun 20, 2015 at 19:57

answered Jun 20, 2015 at 19:09

Andy

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2 Comments

timebandit Over a year ago

Ok almost there. My intention is to modify the Array type so that xadd is available to all instances of array. My initial code was thought to be the way to do this. Manually adding it to instances of arrays is too much work.

Andy Over a year ago

It is available to all instances of array.

timebandit · Accepted Answer · 2015-06-21 11:21:47Z

0

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().

edited Jun 21, 2015 at 11:21

answered Jun 20, 2015 at 22:26

timebandit

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