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I'm trying to interpret this piece of Javascript code. What is the use of the $ and _ signs here? In particular, is $ an alias for the JQuery library, and does this also apply for $set? 

Template.postEdit.events({
  'submit form': function(event) {
    event.preventDefault();

    var currentPostId = this._id;

    var postProperties = {
      url: $(event.target).find('[name=url]').val(),
      title: $(event.target).find('[name=title]').val()
    }

    Posts.update(currentPostId, {$set: postProperties}, function(error) {
      if (error) {
        // display the error to the user
        throwError(error.reason);
      } else {
        Router.go('postPage', {_id: currentPostId});
      }
    });
  }
});
runawaykid
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  • Seems like meteorjs code. Is it ? – Prabodh M Jun 25 '17 at 08:18
  • In both cases, they're just part of the variable name. ie `var _id = 1` and `var $set = postProperties`. – freedomn-m Jun 25 '17 at 08:21
  • @PrabodhM, yup it is. – runawaykid Jun 25 '17 at 08:24
  • @freedomn-m, is there a reason why we use these special characters within variable names in JS? Does JS allow other special characters in variable names besides _ and $? – runawaykid Jun 25 '17 at 08:24
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    Generally, `_` will mean a private variable (hence `this._id). And people frequently (incorrectly imo as it's out-dated hungarian notation) use `var $var=` to mean it's a jquery object. If this is a third-party library (mention of 'meteorjs') then it's up to them how they (badly...) named their parameters. – freedomn-m Jun 25 '17 at 08:26
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    Tried to find an official source for this: *The first character must be a letter, an underscore (_), or a dollar sign ($).Subsequent characters may be any letter or digit or an underscore or dollar sign. (Numbers are not allowed as the first character so that JavaScript can easily distinguish identifiers from numbers.)* – freedomn-m Jun 25 '17 at 08:31

1 Answers1

4

Explaination

$ and _ are mostly appended in Javascript variables to give more readability and to be more distinguishable ( visually most of the times ) from other variables. They are just conventions used by JS developers. Not necessary you've to use them. Major libraries/frameworks like Jquery, Angular like to follow this style in their frameworks.

Usage of $

Jquery have wrapped their features in $ . If you have included jQuery in your application, then $ used alone stands for jquery Object. Jquery being a really popular modern library, have somehow snatched the variable in JS. But its not like they have licensed the variable ( Just think, if variable name could be licensed, development would be more painful than it is now :p ), its more like they have dominated the use of $. 

var selector = $('.someclass'); /* This is jquery object similar to
var selector =  jQuery('.someclass') */

var $somestring = 'some string'; // Here $ character is appended to a variable. 
//It doesn't adds any special behavior to the variable.

Some people have make good use of $, check it out here.

A naive developer who have used too much or the only library as JQuery in his lifespan is prone to this confusion. When he sees source code from framework like AngularJS, he tries to relate things with his former love jQuery. In Angular variables like $scope, $compile, etc, they seem confusing to him, as they have heavily appended $ to name their objects. Its just another name, you can write code with or without it. Angular uses this convention to distinguish variables from local to special objects. Big guns always try to dominate their conventions over the developer community. Can't blame them much, its for the betterment for all

Usage of _

Riding in similar vogue bandwagon, _ was nearly snatched by another useful ( really ? we can live without it ) library called Underscore Js . So they use the _ as their Underscore object, or mostly developers are to be blamed for this abuse, as they have paved its path to the vanity. But we can't blame developers for this, they were just using following good naming conventions.

var _myName = 'Who Cares'; // similar to $ no special behavior 

var currentPostId = this._id; // In your case it seems

Well _ is mostly used by developers to distinguish the variables as private data members of their class, but only naming doesn't guarantee the access level. A good post briefly explaining for this is here

The best part is that, all the special characters that are allowed in Javascript to create distinguishable variable names are already invaded by biggies. So no more confusion. It is understandable why underscore invaded _ as their supremo, it stands for its meaning. But I am still curious why $ was chosen by jQuery. It doesn't even rhyme with it. No distant relation, it seems jQuery just took it as their property. I don't find any post explaining their invasion over it.
Sorry for being so dramatic, comic and sarcastic. Feel free to downnvote if it does not suit your appetite. Here to just help and make this space more interesting.

P.S : A list of valid characters for the naming convention used in JS is explained here

Prabodh M
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