I want to have my own method in JavaScript which can be able to access some property of the preceding method/function Eg:
//default
var arr = [1,6,3,5];
alert(arr.length);
This will alert the length of arr. But I want to use my custom method instead of the length method say len Eg:
//custom
prototype.len = ()=>{
return prototype.length;
}
var arr = [1,6,3,5];
alert(arr.len);
To define a "getter" without using ES6 class syntax you can use Object.defineProperty:
Object.defineProperty(Array.prototype, 'len', {
get: function() {
return this.length;
}
});
Use of .defineProperty will (by default) create a non-enumerable property which ensures that your new property doesn't appear inadvertently in the results of a for .. in ... loop.
The question of whether it's appropriate to add such a property to a built-in class is a matter of some debate. Some languages explicitly encourage it, some don't.
You may define your own method for prototype:
// also you may use Object.defineProperty
Array.prototype.len = function() {
return this.length // --> this will be an array
}
and then call the function
[1, 2, 3].len()
BUT
It is a bad practice to define any new function in built-in prototypes - you can accidentally redefine existing method or method with the same name will be added in the future and it will cause unpredictable behavior.
Better approach either create your own prototype and use it
function MyArray () {
// your code
}
MyArray.prototype = Object.create(Array.prototype) // inheritance
MyArray.prototype.len = function () {
return this.length
}
or just create simple function and pass an array as an argument to it or as this:
as argument:
function len (array) {
return array.len
}
as this:
function len() {
return this.len
}
// invoking function
len.call(array)
method. But it is still bad idea to add anything to built-in prototype :-)You can do it like this :
Array.prototype.len = function () {
return this.length;
};
But there can be a better option, see comments below.
Object.defineProperty(Array.prototype, 'len', { ... }) instead.You can create your custom methods or properties to built-in objects, in the example you gave, you want to alter the Array built-in object and you have to be specific. For instance:
Array.prototype.len = function() { return this.length; };
[1, 2, 3].len(); // this will return 3
Keep in mind that this refers to the array you're calling the method and if you use an ES6 arrow function, the scope will be affected and therefore the this value.
