Javascript is a flexible and quirky programming language that lets you define functions in a number of different ways. The two more common ways to set up a function is using Function Expressions and Function Declarations. Knowing the differences between these two can have a big effect on how you organize your code, especially when it comes to hoisting and scope.

Function Declarations

To declare a function, follow these steps:

  1. Start with the keyword function.
  2. Give your function a name. This is how you will refer to it when calling the function later.
  3. Add parentheses (). If the function needs input values (called parameters), list them inside these parentheses.
  4. Write the function body inside curly braces {}. This is where the code that performs the task goes.
function add(x, y) {
  return x + y;
}
console.log(add(5, 3));  // Output: 8

Characteristics of Function Declarations:

Hoisting: Function declarations are automatically "hoisted" to the top of the file. This means you can call a function anywhere in the JavaScript file, even before its actual definition appears in the code.

Scope: Function declarations are subject to either function scope or global scope, depending on where they are defined:

  • Function Scope: If a function is defined inside another function, it is only accessible within the parent function. It cannot be called from outside the parent function.
  • Global Scope: If a function is defined outside of any other function, it is available anywhere in the entire script and can be called from any part of the code.

Mutability: Once a function is defined, its name is immutable, meaning you cannot change it. The reference to the function name remains fixed and cannot be reassigned to point to a different value or function.

Function Expressions

A function expression involves creating a function and assigning it to a variable. The main difference between a function declaration and a function expression is that the letter does not need a name and is typically used as part of an expression (like assigning it to a variable).

To write a function expression, follow these steps:

  1. Start by defining a variable (using const, let, or var).
  2. Assign an anonymous function (a function without a name) to the variable.
  3. Use parentheses () for any parameters the function needs.
  4. Write the function body inside curly braces {} where you define the task.

const subtract = function(x, y) {
  return x - y;
};
console.log(subtract(10, 4));  // Output: 6

Characteristics of Function Expressions:

  • Hoisting: Unlike function declarations, function expressions are not hoisted. This means you cannot use a function expression before it is defined in the code. If you attempt to call it before its definition, you will encounter a ReferenceError.
  • Scope: Function expressions are scoped to the block in which they are defined, similar to function declarations. They are only accessible within the block where they are created.
  • Mutability: The variable to which the function expression is assigned can be reassigned. You can change the value of the variable to something else, such as another function or a different value.

Conclusion

To truly understand JavaScript at a deeper level, it's essential to have a solid understanding of both function declarations and function expressions. The choice between the two depends on your application's requirements and your coding preferences.Function Declarations are "hoisted," meaning they can be used before they are defined in the code, giving you more flexibility in how you structure your functions.Function Expressions, on the other hand, offer more dynamic possibilities. For example, you can create anonymous functions or define functions conditionally.Understanding the differences between these two is crucial for all JavaScript developers—whether you're a beginner learning about concepts like scope and hoisting, or an experienced programmer managing complex applications.

Last Update: October 15, 2024