CODEWITHSUNDEEP
javascript
Will
Will

Reputation: 681

how to write a class with an attribute, and when new an object, this attribute can self-increase

I would like to write a student class with an id attribute

when I new objects like this:

var s1 = new Student()
var s2 = new Student()
var s3 = new Student()

I can get this result:

console.log(s1.id) //1
console.log(s2.id) //2
console.log(s3.id) //3

Upvotes: 1

Views: 56

Answers (2)

andars
andars

Reputation: 1404

You could do this with either a global variable or a static variable (The latter is preferred). This means that the variable belongs to the type rather than the instance. 

function Student(){ 
    this.id = Student.currentId;
    Student.currentID++;
}
Student.currentID = 1;

Upvotes: 2

cookie monster
cookie monster

Reputation: 10972

var Student = (function() {
    var id = 0;

    return function Student() {
        this.id = ++id;
    };
})();

Here every new Student object will get an incremented id from the var id that is in the outer IIFE.

The IIFE part isn't strictly needed, but it protects the var id from being accidentally modified by other code. Without the IIFE, it would simply be:

var id = 0;

function Student() {
    this.id = ++id;
}

But the var id can be reached by any code in the enclosing scope, so the first example is safer.

Upvotes: 4

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