JS Visualizer – Visualize Context, Hoisting, Closures, and Scopes in JavaScript

Re:JS Visualizer – Visualize Context, Hoisting, Closures, an... by zawerf on Tuesday October 09, 2018 @09:37PM (#18181319)

ES5 has a lot of interesting hoisting bugs.

For example this doesn't print what you expect:

  for (var i = 0; i < 3; i++) {
    var text = 'number ' + i;
    setTimeout(function() {
      console.log(text);
    }, i * 1000);
  }

But flipping var to let will fix it. The bug is that var is not block-scoped so everything refers to the same `text` variable which gets mutated by the next iteration.

I was hoping their demo would be good for explaining this bug but it seems to be broken.

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Re:JS Visualizer – Visualize Context, Hoisting, Closures, an... by jazzyjackson on Tuesday October 09, 2018 @10:07PM (#18181453)

This is not a bug, this is spec.

variables in javascript are not block scoped, but functionally scoped.

the let keyword introduces block scope to javascript.

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