Exploration of var vs const in NodeJS Application

Variables are a fundamental part of any programming language, including TypeScript. In TypeScript, developers have the option to use var and const to declare variables. While var has been used in JavaScript for a long time, const was introduced with the ES6 standard and has quickly become a popular choice for declaring variables. In this article, we will explore the performance benefits of using var and const in an Express.js TypeScript application.

The Difference Between var and const

Before we dive into the performance benefits, let's take a look at the differences between var and const:

var: Variables declared with var are function-scoped, meaning that they are accessible within the function they are declared in. They can also be re-declared and re-assigned.

const: Variables declared with const are block-scoped, meaning that they are only accessible within the block they are declared in. They cannot be re-declared, but they can be re-assigned if they are mutable.

Now that we understand the differences between var and const, let's explore their performance benefits.

Performance Benefits of Using const

One of the main performance benefits of using const is that it allows the TypeScript compiler to optimize the code better. When a variable is declared with const, the compiler can determine that it will never change, and can optimize the code accordingly.

For example, consider the following code:

let myNumber = 10;
for (let i = 0; i < 1000000; i++) {
  myNumber += i;
}
console.log(myNumber);

In this code, myNumber is declared with let, which means that it can be re-assigned. As a result, the TypeScript compiler cannot determine if myNumber will change or not. This can lead to slower code execution.

Now let's take a look at the same code, but with myNumber declared with const

const myNumber = 10;
for (let i = 0; i < 1000000; i++) {
  // This will throw an error because myNumber is read-only
  myNumber += i;
}
console.log(myNumber);

In this code, myNumber is declared with const, which means that it cannot be re-assigned. As a result, the TypeScript compiler can determine that myNumber will never change, and can optimize the code accordingly. This can lead to faster code execution.

In fact, according to a benchmark test conducted by Stefan Baumgartner, using const can be up to 95% faster than using let in some cases.

Performance Benefits of Using var

While const can provide significant performance benefits, there are also cases where var can be more performant. One such case is when using closures.

Consider the following code:

function counter() {
  let count = 0;
  return function() {
    return ++count;
  }
}

const myCounter = counter();
console.log(myCounter());
console.log(myCounter());
console.log(myCounter());

In this code, count is declared with let. While this code works correctly, it is not as performant as it could be. This is because each time the function returned by counter is called, a new instance of count is created. This can lead to slower code execution.

Now let's take a look at the same code, but with count declared with var:

function counter() {
  var count = 0;
  return function() {
    return ++count;
  }
}

var myCounter = counter();
console.log(myCounter());
console.log(myCounter());
console.log(myCounter());

In this code, count is declared with var. Because var is function-scoped, count is only created once, and subsequent calls to the function returned by counter reuse the same count variable. This can lead to faster code execution.

Conclusion

In conclusion, when it comes to performance in an Express.js TypeScript application, using const can provide significant benefits in most cases. However, there are some cases where var can be more performant, such as when using closures. It's important to understand the differences between var and const and choose the appropriate one for each use case to achieve optimal performance.

While the performance differences between var and const may not be noticeable in small applications or single requests, they can add up over time and become a significant factor in larger applications or high-traffic scenarios. As such, it's worth taking the time to consider which one to use for each variable declaration in your Express.js TypeScript application.