The JavaScript let and const keywords provide block-level scope, but there is a slight difference in how they behave. With const, you can not re-assign a value to the variable. With let,
you can.
Over time, JavaScript applications have grown in complexity. As a result of the increased code complexity programmers have been faced with a challenging dilemma: build applications that satisfy ever-evolving business requirements, yet continue to work with the same tools. It only makes sense that JavaScript would be in need of improvements, since for much of its history, functions were the only tools available to achieve scope. But, for several years, block-level scope was a feature that was sorely lacking. Then along came the ECMAScript-2015 specification that finally met that need with the let and const keywords.
The JavaScript let and const keywords are quite similar, in that they create block-level scope. However, they do differ a bit in the way that they behave. For example, the JavaScript let keyword is similar to the var keyword in that assignments can be changed. On the other hand, the JavaScript const keyword differs in that assignments can not be changed. So, once you declare a variable using the const keyword, you cannot re-assign a value to that variable. This does not mean that the variable is immutable. If you assign an object to a variable using the const keyword, you can mutate that object. You just can’t re-assign that variable with a new value. Let’s take a look at some examples.
Example # 1 A
Example # 1 B
In Example # 1 A, we have two different versions of the “i” variable. I say “two different versions” because the same variable name exists in two difference scopes, the global scope and a block scope. The block scope exists between the two curly braces: “{ }”. Then inside of the two curly braces, I used the JavaScript let keyword to declare a second “i” variable. Because we used the let keyword, that particular “i” variable is scoped to the block in which it was declared. And because of this, the console.log() statement on line # 6 outputs 50. I’ll just note here that it may seem a little odd at first to declare a variable anywhere other than at the top of the function, but this actually is the correct syntax; if we want a block-level scope variable, we use the let keyword inside of a set of curly braces.
Take a look at Example # 1 B. Notice how, in the second console.log() statement, the output is 100. This is because that second console.log() statement is in the global scope, and in that scope the “i” variable is equal to 100. So, there we have it: two different scopes without even having used a function.
Example # 2 A
Example # 2 B
Now, in Example # 2 A, there are two “j” variables.
The first “j” variable is a global, equal to 100, and the second is defined inside of the for loop. And because it’s defined inside of a block, it has block-level scope. Now look at example # 2 B. Because “i” is global, the “i” variable increments, just as we would expect. But notice that the “j” variable is always 50 in each console.log() statement, even though there is a global “j” variable. This is because on each iteration of the for loop, a block-level “j” variable is declared using the let keyword, and it is incremented (just to demonstrate that with let, we can re-assign a variable value). So in this case, with each iteration of the for loop we have a block-scoped “j” variable and it is always 51. Note that the global “j” variable is ignored on line # 12.
Example # 3 A
Example # 3 B
In Examples # 3 A and # 3 B you’ll see a similarity to Examples # 1 A and # 1 B, the only difference being the use of the use of the const keyword instead of let when declaring our block-level version of the “i” variable.
Example # 4 A
Example # 4 B
Now here in Example # 4 A, we’ve run into a problem. We tried to take the same approach as Example # 2 A, that is, we tried to increment the “j” variable declared on line # 6. The problem, though, is that when you use the JavaScript const keyword, you cannot re-assign a new value to a variable. So when you look at Example # 4 B, you’ll see that we never see the full output of the for loop that we expected, because line # 9 of Example # 4A throws a TypeError. This is because when we try to change the value of “j”, we find that this is not possible because it was created using the const keyword. In other words: it’s a constant.
Example # 5 A
Example # 5 B
Now Example # 5 A is virtually identical to Example # 4 A, except that we have not tried to increment the “j” variable. And when you look at Example # 5 B, you’ll see that we no longer have an error. In the console, the value of “j” is 50 every time.
Summary
So to recap, we now know that the JavaScript let and const keywords allow you to create block-level scope for variables, which, in turn, negates the need to always use functions to create scope. With block-level scope, all you need are the curly braces “{ }”, and within that block, any variable created using let or const is private (or local) to that block. This is particularly helpful with for-loops. And a very important thing to keep in mind: with const, you cannot re-assign values to a variable. In other words, any variable created with the const keyword is a constant and the assignment cannot be changed.
A lot to take in here, but I think it’s worth keeping on your radar, given this very functional block-level scope now increasingly available in browsers.
