ElectronJs: What does curly braces '{}' mean in package json - json

Question

I was going through some electron package.json examples where I found some interpolations like given below:
"updater": {
"urls": {
"darwin": "{{& SQUIRREL_UPDATES_URL }}/update/%CHANNEL%/darwin?version=%CURRENT_VERSION%",
"win32": "{{& SQUIRREL_UPDATES_URL }}/update/%CHANNEL%/win32",
"linux": "{{& SQUIRREL_UPDATES_URL }}/update/%CHANNEL%/linux"
}
}
"piwik": {
"serverUrl": "{{& PIWIK_SERVER_URL }}"
},
"sentry": {
"dsn": "{{& SENTRY_DSN_PRIVATE }}"
}
I do not really know the following:
what does this {{}} mean in json
where does these variable exist
what does & mean in {{}} "{{& SENTRY_DSN_PRIVATE }}"
If anyone can explain then it would be really kind. Many thank in advance.

Answer 1

I guess you are talking about Whatsie and it's package.json.
If you take a look at one of the Gulp tasks located in the file tasks/compile.coffee, you'll be able to see the lines (in CoffeeScript):
# Move package.json
gulp.task 'compile:' + dist + ':package', ['clean:build:' + dist], ->
gulp.src './src/package.json'
.pipe mustache process.env
.pipe gulp.dest dir
Here the actual package.json is being passed to a mustache template engine - it receives a template as a first argument (package.json here acts like a template) and a data to be inserted in the template as a second argument - process.env.
As package.json acts like a template for mustache, you can use mustache syntax in it.
Curly braces {{}} are the part of it, they are used as placeholders which will be replaced by the actual data, when templates are being compiled. In the mustache docs you can also find a line:
You can also use & to unescape a variable: {{& name}}
So {{& name}} is to prevent values from being escaped. Otherwise, if you don't use & and values for output have some dangerous characters , they will be replaced by more secure ones (originally to prevent XSS in templates), as a result it will transform initial value, which is not always what you want. In this case author wants to preserve original value.
Going back to process.env - it is an object which gives access to environment variables in Node.JS. There is a file in repository .env-example with an example of env variables developer has to set in order to have the application work differently in different environments (for example on local machine or CI server). Names of some of the variables in this file are the ones that are used in a package.json as template placeholders - I guess author of the app uses all of this to simplify a build process for different environments.