The var and let keyword can get very confusing and may lead to unexpected behaviors in an application and may also lead to some silly bugs. I will explain what is both and when to use one.

Take a look at the code below,

What is the expected output? I know, you might well say this code prints undefined in the last line. I also thought so. But no. The output of the following code is:

  

There is a simple explanation for it. A variable declared with the var keyword is accessible to the whole function in which it is declared rather than just the block. In the code above, although the variable i is declared inside the for loop, it is available outside it also as the for loop itself is inside the function do(). This is called variable hoisting in JS. In other programming languages such as Java or C++ etc, this code will throw a compile-time error immediately. We need to be very careful with the usage of the var keyword.

The code is written with let keyword,

While the variable i declared with the let keyword behaves properly in the case above. It will throw an error in console.log("Final value of i" + i); line if we compile this code with Typecript.

My thoughts with the usage of both should be known first. We should use the let keyword in most of our variable declarations. var can be used in the scenarios where you want the variable to access inside the whole function.