Exploration of var vs const in NodeJS Application
Published: 2023-03-01 04:53:07

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:
``` typescript
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`

``` typescript
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:
``` typescript
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`:
``` typescript
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.