What is the keyword used to declare a constant in JavaScript?
In JavaScript, the keyword used to declare a constant is "const". The "const" keyword is used to define a variable that cannot be reassigned after it has been initialized. It is commonly used to declare values that are intended to remain constant throughout the execution of a program.
To declare a constant using the "const" keyword, you simply use the keyword followed by the variable name and the initial value. For example:
javascript const PI = 3.14159;
In the above example, the constant "PI" is declared with the value of 3.14159. Once the constant is declared, any attempt to reassign a new value to it will result in an error. For instance, the following code will throw an error:
javascript const PI = 3.14159; PI = 3.14; // Error: Assignment to constant variable
Constants are block-scoped, which means they are only accessible within the block they are defined in, similar to variables declared with the "let" keyword. This allows for better control and encapsulation of the constant values within specific scopes. For example:
javascript
function calculateArea(radius) {
const PI = 3.14159;
const area = PI * radius * radius;
return area;
}
console.log(calculateArea(5)); // Output: 78.53975
console.log(PI); // ReferenceError: PI is not defined
In the above code, the constant "PI" is declared within the scope of the "calculateArea" function and is only accessible within that function. Any attempt to access it outside the function will result in a reference error.
It is worth noting that while constants cannot be reassigned, the value they hold can still be mutable. For example, if a constant holds an object or an array, the properties or elements of the object or array can be modified. However, the constant itself cannot be reassigned to a new object or array. For example:
javascript
const person = {
name: "John",
age: 30
};
person.age = 31; // Valid
person = { // Error: Assignment to constant variable
name: "Jane",
age: 25
};
In the above code, the properties of the "person" object can be modified, but the constant itself cannot be reassigned to a new object.
The "const" keyword is used to declare a constant in JavaScript. Once a constant is declared, its value cannot be reassigned. Constants are block-scoped and can be used to encapsulate values within specific scopes. While the value held by a constant can be mutable, the constant itself cannot be reassigned to a new value.
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