const over let unless I'm forced to

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.

Comments

[Andrew Lee]

Thanks for pointing that out. I just made an edit to the post to address this.

I still would recommend using const for 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.

const c = { b: 1 };
c = {...c, b: 42};

What's most important is for the team members to be on the same page and use linters if possible to codify the understanding.

[dragonDScript]

var is underrated.