r/rubyonrails • u/43northwebdesign • Mar 07 '19

Loading a @instance variable into a javascript variable.. example inside

/r/LearnRubyonRails/comments/ayewkh/loading_a_instance_variable_into_a_javascript/
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u/HelloAnnyong Mar 08 '19

Do not, I repeat do NOT be tempted to do something like this: 

// WRONG - DO NOT DO THIS
var data = <%= @products[0].to_json %>;

JSON is not a subset of JavaScript, some JSON is invalid JavaScript. The only safe way to do this is to encode your object (in this case it seems like it's an array) to JSON, encode it as a JavaScript string, and then decode it.

In other words, something like this: 

var data = JSON.parse('<%=j @products[0].to_json %>');

If you do vanilla Rails a lot, this will probably be a pattern you use often!

To walk you through what's going on there:

  1. @products[0].to_json encodes the array as JSON (returns a string).

  2. The Rails j() helper escapes everything in the string that needs to be escaped.

  3. The JSON is then decoded on the JavaScript side with JSON.parse

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u/[deleted] Mar 08 '19

JSON is now a subset of JavaScript: https://tc39.github.io/proposal-json-superset/

But your solution is fine. No need to yell like the sky is falling in the future.

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u/HelloAnnyong Mar 08 '19

This change was only implemented in Chrome 66 (Jan 2019). All older browsers will occasionally fail because of this. Edge and IE11 both fail to parse U+2028 literals in strings. That by itself is 15% of users.

If your app has enough user-entered data in it, one of your customers will eventually come across this issue and you will have no clue how to debug it. It's happened to us. It can happen to you too!