大括号内的JS变量定义

Question

I was taking a look to an old javascript file and I noticed a function like this:

function mainFunction() {

  var foo = new objFoo();

  foo.name = "fooname";
  foo.desc = "foodesc";

  // some instructions here

  {

    var bar = new objBar();

    bar.name = foo.name;
    bar.desc = foo.desc;

    // some other instructions here

  }

  return(foo);

}

My question is: what is the purpose (if there is one) of those curly braces surrounding the bar object definition?

Thanks

Answer 1

Curly braces like that in JavaScript define a block . They can be used to contain scope if a variable is initialised using either let or const , but since the variable in your example is initialised using var , they don't actually do anything (unless there's a statement in the preceding line like an if or while etc.).

Just going by the code you've given us, I think it's a mis-interpretation of code or a mis-understanding of JavaScript by the developer. To contain scope in JavaScript using var variables, you'll need to write closure like this:

function mainFunction() {

  var foo = new objFoo();

  foo.name = "fooname";
  foo.desc = "foodesc";

  // some instructions here

  (function () {

    var bar = new objBar();

    bar.name = foo.name;
    bar.desc = foo.desc;

    // some other instructions here

  }());

  return(foo);

}

Answer 2

The curly braces define a block with a local scope. You may define variables using const or let in the block the scopes of which are limited to the block.

For example:

 const a = "world" { const a = "hello" console.log(a) } console.log(a) 

Using var inside the block of curly braces is dangerous since it overwrites variables with the same name that are defined outside the block. If you use var declarations instead of const / let , the only advantage of a block can be a better legibility.

 var a = "world" { var a = "hello" console.log(a) } console.log(a)