In JavaScript, a data type is a classification of the type of value that a variable or expression can hold. There are several data types in JavaScript, including:

Numbers: for numeric values.
For example:

let x = 5;

Strings: for textual data.
For example:

let name = "John Doe";

Booleans: for true/false values.
For example:

let isValid = true;

Null: for null values, which represent the absence of a value.
For example:

let x = null;

Undefined: for variables that have been declared but not yet assigned a value. For example:

let x;
console.log(x); // Output: undefined

Object: for complex data structures.
For example:

let user = {
  name: "John Doe",
  age: 30
};

Note that the data types in JavaScript are dynamic, which means that a variable can hold values of different data types at different times.
For example:

let x = 5;
console.log(typeof x); // Output: number

x = "Hello";
console.log(typeof x); // Output: string

In the code above, the typeof operator is used to check the data type of the x variable.
At first, x is a number, but after it is assigned the string value "Hello", its data type becomes a string.