Why prefer-const
One Way to Do It: It is mental overhead to have to choose between let and const every time. A rule like “always use const where it works” lets you stop thinking about it and can be enforced by a linter.
Reassignments May Cause Bugs: In a longer function, it can be easy to miss when a variable is reassigned. This may cause bugs. Particularly in closures, const gives you confidence you’ll always “see” the same value.
Learning About Mutation: Folks new to JavaScript often get confused thinking const implies immutability. However, one could argue that it’s important to learn the difference between variable mutation and assignment anyway, and preferring const forces you to confront this distinction early on.
Meaningless Assignments: Sometimes, an assignment doesn’t make sense at all. For example, with React Hooks, the values you get from a Hook like useState are more like parameters. They flow in one direction. Seeing an error on their assignment helps you learn earlier about the React data flow.
Performance Benefits: There are occasional claims that JavaScript engines could make code using const run faster due to the knowledge the variable won’t be reassigned.
Why Not prefer-const
Loss of Intent: If we force const everywhere it can work, we lose the ability to communicate whether it was important for something to not be reassigned.
Confusion with Immutability: In every discussion about why you should prefer const, someone always confuses with immutability. This is unsurprising, as both assignment and mutation use the same = operator. In response, people are usually told that they should “just learn the language”. However, the counter-argument is that if a feature that prevents mostly beginner mistakes is confusing to beginners, it isn’t very helpful. And unfortunately, it doesn’t help prevent mutation mistakes which span across modules and affect everyone.
Pressure to Avoid Redeclaring: A const-first codebase creates a pressure to not use let for conditionally assigned variables. For example, you might write const a = cond ? b : c instead of an if condition, even if both b and c branches are convoluted and giving them explicit names is awkward.
Reassignments May Not Cause Bugs: There are three common cases when reassignments cause bugs: when the scope is very large (such as module scope or huge functions), when the value is a parameter (so it’s unexpected that it would be equal to something other than what was passed), and when a variable is used in a nested function. However, in many codebases most variables won’t satisfy either of those cases, and parameters can’t be marked as constant at all.
No Performance Benefits: It is my understanding that the engines are already aware of which variables are only assigned once — even if you use var or let. If we insist on speculating, we could just as well speculate that extra checks can create performance cost rather than reduce it. But really, engines are smart.
Top comments (2)
This is actually my main argument for prefer-const. If you always use
const, and you get into the habit of assuming variables will never be reassigned, then anytime you useletot stands out like a beacon and makes the reader pay attention to that variable and watch for it to change. To me this is more important, since variables that change need a closer eye than those that don't.You are totally correct. But I have written about why some people not prefer const. Opinions differ from person to person. :)