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javascriptscopevarlet

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

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)?

Solution

  • 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 ...