CODEWITHSUNDEEP
javascript
user5752691
user5752691

Reputation: 31

Can someone explain this weird behaviour in Javascript?

In this code:

function calc(){
    this.a = 5;
}

calc(); // Run the function once

// Redefine the function
function calc(){
    return this.a;
}

// Re-run the function
calc();
// 5

From a guy coming from Java background.

  1. How is this.a valid (if a hasn't been defined before?) From what I know, this.a should refer to the instance variable a of the class ?

  2. Why did it return 5?

Upvotes: -1

Views: 59

Answers (3)

Mitja Kramberger
Mitja Kramberger

Reputation: 270

"this" value depends on how function was called. Inside a global function (like your example) value of "this" is equal to window object. Which means your two functions are just working with window.a (which will in your case have a value of 5).

Here you can find more information https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this

Upvotes: 2

Paul
Paul

Reputation: 36349

@bknights is correct. To get the behavior you're expecting, you need to new the function.

function calc(){
   this.a = 5;
}

var c = new calc();
console.log(c.a); // will return 5.

c.a = 10;
console.log(c.a); // will return 10

var b = new calc();
console.log(b.a); // will return 5 again

Upvotes: 0

bknights
bknights

Reputation: 15447

I'm thinking you must have run your first calc before you defined and ran your second calc. In the first one you are initializing member a of the global object. In the second you are returning global.a (the global object is generally the window if you are running on a browser).

You have no class defined

Upvotes: 2

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