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In JavaScript, variables are used to store data or values. To use a variable, you must first declare it and then optionally initialize it with a value. Here's how you can declare and initialize variables in JavaScript:

**1. Declaring a Variable:**
To declare a variable, use the `var`, `let`, or `const` keyword, followed by the variable name. The syntax is as follows:

```javascript
var variableName;
let variableName;
const variableName;
```

- `var`: Declares a variable with function scope or global scope (if declared outside a function). However, it is best to use `let` or `const` instead of `var`, as `var` has some quirks due to its function-level scope and hoisting behavior.
- `let`: Declares a block-scoped variable that can be reassigned to a new value.
- `const`: Declares a block-scoped constant variable. Once initialized, its value cannot be changed.

**2. Initializing a Variable:**
After declaring a variable, you can optionally initialize it with a value. The initialization assigns a value to the variable. The syntax for initializing variables is as follows:

```javascript
var variableName = value;
let variableName = value;
const variableName = value;
```

- `value`: The initial value assigned to the variable.

Examples:

```javascript
// Declaring and initializing variables
var age = 30;
let name = "John";
const PI = 3.14;
```

In this example, we declared and initialized three variables: `age`, `name`, and `PI`. The variable `age` is declared using `var`, `name` is declared using `let`, and `PI` is declared using `const`. The variables are assigned values `30`, `"John"`, and `3.14`, respectively.

Remember:

- Variables declared with `var` have function scope or global scope, while variables declared with `let` or `const` have block scope (e.g., within a function or inside curly braces `{}`).
- `let` allows reassignment of values, while `const` does not; its value remains constant throughout its scope.
- It's a good practice to use `let` when you need to reassign a variable's value and `const` when you want to ensure the variable's value remains constant.

If you use `let` or `const`, it's a good habit to initialize the variable with a value at the time of declaration. For variables declared with `var`, if they are not initialized at the time of declaration, they will have an initial value of `undefined`.