In the beginning, this discussion wasn't even necessary. We only had one choice in JavaScript to store value (var). Nowadays I always reach for const unless I'm forced to.
This argument isn't as strong for JavaScript, since there are no known performance gains at the moment of using a constant (const) over a variable (let). This is not the case in other languages (i.e. Swift) and maybe JavaScript will have performance gains for constants in the future.
When am I forced to use let? If I find myself changing a previously declared constant, JavaScript will throw an error.
const x = "hi";
x = "bye";
// VM226:1 Uncaught TypeError: Assignment to constant variable
This way I don't bother with thinking about which declaration I should use, I'll be alerted when I have to by following a rule.
Arrays and Objects
What about arrays and objects? When we mutate an existing object or array, JavaScript will not give us a warning. This is because they are passed by reference.
In this case I still use const even if changes are being made.
const arr = [];
arr.push(1);
When I create a new array with the new changes (common practice in React to trigger state change), JavaScript will throw me a warning.
let arr = [];
arr = [...arr, 1];
In this case I will switch my const to a let.
Conclusion
const over let unless I'm forced to
I just let my tools let me know if a variable is necessary. It also makes sense to use a variable (let) when the value stored inside of it varies throughout the lifecycle of the program.
Top comments (2)
Thanks for pointing that out. I just made an edit to the post to address this.
I still would recommend using
constfor objects and arrays since mutating an existing array or object. This way I can stick with this rule. When I create a new object after mutating it (common in React), JavaScript will force me to use a variable.What's most important is for the team members to be on the same page and use linters if possible to codify the understanding.
var is underrated.