In JavaScript, variables can be declared using the keywords var, let, or const. var is used for variables that can be reassigned, let is used for variables that can be reassigned within a specific scope, and const is used for variables that cannot be reassigned.

In JavaScript, the Number data type is used to store whole numbers.

In JavaScript, Array is not a data type but a built-in object.

In JavaScript, the String data type is used to store text.

In JavaScript, % is not a valid operator.

In JavaScript, the + operator is used for addition. Therefore, the result of the expression 10 + 20 is 30.

In JavaScript, the - operator is used for subtraction. Therefore, the result of the expression 10 - 5 is 5.

In JavaScript, the * operator is used for multiplication. Therefore, the result of the expression 10 * 2 is 20.

In JavaScript, the / operator is used for division. Therefore, the result of the expression 10 / 2 is 5.

In JavaScript, the % operator is used for modulus. The modulus operator returns the remainder of the division of two numbers. Therefore, the result of the expression 10 % 3 is 1.

In JavaScript, the Boolean data type is used to store true or false values.

In JavaScript, the && operator is used for logical AND. The logical AND operator returns true if both operands are true, and false otherwise. Therefore, the result of the expression true && false is false.

In JavaScript, the || operator is used for logical OR. The logical OR operator returns true if either operand is true, and false otherwise. Therefore, the result of the expression true || false is true.

In JavaScript, the ! operator is used for logical NOT. The logical NOT operator returns true if the operand is false, and false otherwise. Therefore, the result of the expression !true is false.

In JavaScript, the ! operator is used for logical NOT. The logical NOT operator returns true if the operand is false, and false otherwise. Therefore, the result of the expression !false is true.