JavaScript Fundamentals: Mutable let,Comments, Functions
Tags: javascript, tutorial, 100daysofcode, webdev
Published: 2022-12-16 15:56:49

Today is the 2nd day of my **#100DaysOfCode** journey with JavaScript.

I am going to write about my learnings in an explained way through my [blogs](https://astrodevil.hashnode.dev/) and socials. If you want to join me on the learning journey, make sure to follow my blogs and social and share yours too. **Let's learn together!🫱🏼‍🫲🏼**

This Article is a part of the [JavaScript Fundamentals](https://astrodevil.hashnode.dev/series/js-fundamentals) series**.**

### Mutable let

We worked on variables in the last section and now going further with variables, how can we change the stored value inside of a variable?

We used `const` keyword to declare a constant variable, but if we try to change the value it will give errors.

```javascript
const a = 4;
a = 8;
```

If we try to run above the line, it will give `TypeError: Assignment to constant variable` . Constants are immutable, meaning their value cannot change.

Here comes the keyword `let` which allows the value to be mutable (meaning it can change).

```javascript
let a = 4;
a = 8;
```

The above line will run without error.

### Comments

Comments are an important part of programs. Writing good comments is also necessary to be a good programmer and to write clean code.

Coming up with good variable names is also important.

```javascript
const value = 99;
const price = 99;
```

Both hold the same value `99` but `price` is more descriptive.

Comments are only for humans to understand code in a better way. We can write single-line or multi-line comments.

```javascript
// this is price in U.S. Dollars
const price = 99;
```

```javascript
/* The price of all items
   Denominated in U.S. Dollars  */
const price = 99;
```

### Functions

A function is a reusable code and it returns an output. The function must be defined before calling it. Function call means to execute the function. The word *invoke* is also used and the meaning is the same as call.

When a function is called, It means you're passing specific input values.

```javascript
const output = addOne(5);
```

In the code above, `addOne` is the function and parenthesis `( )` is used to call the function. We are passing in an input value 5 into our function addOne.

It is not mandatory to have input values in the function. See the below example.

```javascript
const message = getMessage();
```

In order to call a function, we must first define it!

```javascript
function addOne(input) {
    return input + 2;
}
```

In the above code, we are creating a function called `addOne` which takes one input called input. We are returning the input plus two.

```javascript
function getMessage() {
    return "Hello World!";
}
```

In the above code, function `getMessage` does not take input. We are simply returning a string, saying "Hello World!". The `return` statement will return the desired output of the function.

```javascript
const a = addOne(2);
```

Variable `a` is now assigned to the return value of the `addOne` function invoked with an input of 2, which evaluates to 4.

### Conclusion

Ending with an extra bit of information about JavaScript functions...

If we define/declare a function once, it can be called elsewhere in the program.

**Today I learned about Mutable let, Comments, and Functions in JavaScript.**

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