Question

I didn’t take any official course of Javascript.

I often read this in tutorials:

function Bank(name) {
   this.name = name;
   var balance = 100;

   // *my question here* method to access balance
   this.getBalance = function() {
       return balance * 0.9 ;
   };
}

var myBank = new Bank("My Bank");
console.log( myBank.getBalance() );

Why do developers use getBalance() instead of getBalance?

I think “getBalance” is easier to read, but no tutorial use this:

// *my question here* method to access balance
   this.getBalance = balance * 0.9 ;

then

console.log( myBank.getBalance );

I didn’t see any tutorial to write using above style, even it is simpler. So:

Is there any different between 2 style of codes? Is there any concern to avoid .getBalance to operate the private variable?

Thanks in advance to explain.

Answer 1

You must understand the different between variable, function and function call. This is variable:

this.getBalance = balance * 0.9 ;

The value of getBalance variable is computed once when the code is executed. Every time you access getBalance (without parentheses), previously computed value is returned.

There is an important implication of using getBalance variable. If you modify underlying balance variable in the future, getBalance won’t reflect that change because it was computed using old balance value.

In the case below:

this.getBalance = function() {
   return balance * 0.9 ;
};

getBalance is also variable but it points to a function, not to a simple value. If you access it like variable, you will get a function reference:

var fun = this.getBalance

You can now call that function using fun(), but fun reference itself (same as this.getBalance) isn’t very useful in your case.

And of course you can call the function by using this.getBalance(). The semantic difference is as follows: every time you call getBalance(), the body of the function is executed. This means the value of balance is read again and getBalance() return value is always up to date.

As you can see there is a huge difference between assigning a function and assigning a simple value (like number) to a variable. Depending on which approach you choose, it will have different implications. You must learn about functional side of JavaScript in order to fully utilize its power.