How do you use the typeof operator in JavaScript?

The typeof operator in JavaScript is used to determine the type of a variable or expression. It returns a string indicating the data type of the operand. It is a unary operator that can be used with any operand in JavaScript.

Here are some examples of how to use the typeof operator in JavaScript:

1. Using typeof with a number:

let num = 42;
console.log(typeof num); // Output: "number"

In the above example, the typeof operator is used to determine the data type of the variable num, which is a number. The console.log statement will output "number".

2. Using typeof with a string:

let str = "Hello, world!";
console.log(typeof str); // Output: "string"

In the above example, the typeof operator is used to determine the data type of the variable str, which is a string. The console.log statement will output "string".

3. Using typeof with a boolean:

let bool = true;
console.log(typeof bool); // Output: "boolean"

In the above example, the typeof operator is used to determine the data type of the variable bool, which is a boolean. The console.log statement will output "boolean".

4. Using typeof with null:

let nullValue = null;
console.log(typeof nullValue); // Output: "object"

In the above example, the typeof operator is used to determine the data type of the variable nullValue, which is null. However, typeof will incorrectly return "object" for null. This is a known bug in JavaScript and has been around since the language’s inception.

5. Using typeof with undefined:

let undefinedValue;
console.log(typeof undefinedValue); // Output: "undefined"

In the above example, the typeof operator is used to determine the data type of the variable undefinedValue, which is undefined. The console.log statement will output "undefined".

6. Using typeof with an object:

let obj = { name: "John", age: 30 };
console.log(typeof obj); // Output: "object"

In the above example, the typeof operator is used to determine the data type of the variable obj, which is an object. The console.log statement will output "object".

7. Using typeof with a function:

function add(a, b) {
  return a + b;
}
console.log(typeof add); // Output: "function"

In the above example, the typeof operator is used to determine the data type of the function add. The console.log statement will output "function".

8. Using typeof with an array:

let arr = [1, 2, 3];
console.log(typeof arr); // Output: "object"

In the above example, the typeof operator is used to determine the data type of the variable arr, which is an array. However, typeof will return "object" for arrays.

In conclusion, the typeof operator is a useful tool for determining the data type of a variable or expression in JavaScript.

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