[Javascript] You can't pass a variable

Let's see a simple code example:

let pet = "dog"
console.log(pet)

We have console.log(pet) inside the code, when you read it, you might say:

Pass ´pet´ variable to console log function

Well, actually it is wrong.

In Javascritp, we cannot pass a variable around, we actually pass a value.

When javascript read that code, seeing 'pet', it looks for its value first. Then pass the value (which is 'dog') to the console log.

So to remember in Javascript, we don't pass variable, we pass value.

Why we need to understand those?

Let's see another example:

let x = 10;
let y = x;
x = 0;

What's the value of ´y´?

The Answer is 10.

So, in line 1: we assign variable x to the value of 10.   x ---> 10

line 2: we assgin variable y to the value of x which is 10. It is important here, we didn't assign variable x to the y, we assign value to the y.  y ---> 10

line 3: we assign variable x to the value 0.  x---> 0

Therefore, y still holds the value of 10. 

If you got the answer is 0. Then you might made the mistake, when you see `let y = x`; you might think we assign variable 'x' to 'y', if x changes, y also changes. THAT'S WRONG. Remember, We don't pass variable, we pass value.

Exercises:

'feed' function locates in another file, maintained by other people. Is that possible just modify 'feed' function, so that console.log(pets[0]) would output a different value other than 'D'?

let pets = "Dog and cat";
feed(pets);
console.log(pets[0]); // D

Answer: Not possible. We pass value 'Dog and cat' to the function. It is String type, belongs to primitive type, which is immutable, You cannot mess up with that.

---

'feed' function locates in another file, maintained by other people. Is that possible just modify 'feed' function, so that console.log(pets[0]) would output a different value other than 'Dog'?

let pets = ["Dog", "Cat"];
feed(pets);
console.log(pets[0]); // Dog

Answer: Yes, it is. We still pass the value, not variable to the feed function, but Array is not primitive value, they are 'object'. So you can mutate the value inside function, so it will output soemthingelse other than 'Dog'