What is a real-world example of block scope being useful in JavaScript?

Question

let and const have introduced block-level scoping to JavaScript, and I now understand the difference. But I'm yet to find a compelling real-world example where let has an advantage over var.

What does block-scoping offer in practical terms (illustrated by example)?

https://github.com/getify/You-Dont-Know-JS – t3__rry May 16 '18 at 07:55 — t3__rry, May 16 '18 at 07:55
@t3__rry this is a great resource, thanks – DavidR May 16 '18 at 10:18 — DavidR, May 16 '18 at 10:18

score 0 · Accepted Answer · answered May 16 '18 at 07:55

1) It creates consistency with other languages:

With var it is often quite unclear where a variable is defined and where we can access it and were not:

 if(false) {
    var a = 1;
 } else {
    a = 2; // wtf
 }
 alert(a);

With let its obvious:

let a;
if(false) { a = 1; } else { a = 2; }

2) It allows useful closuring in for loops:

 for(var i = 0, i < 10; i++)
   setTimeout(() => console.log(i), 100);
   // 10 10 10 10 ...

 for(let i = 0, i < 10; i++)
   setTimeout(() => console.log(i), 100);
   // 0 1 2 3 4 5 6 ...

What is a real-world example of block scope being useful in JavaScript?

1 Answers1