Nunjucks doesn't have a comma filter! This fixes that.
npm install --save-dev nunjucks-comma-filter
var nunjucks = require('nunjucks');
var commaFilter = require('nunjucks-comma-filter');
// However you end up with a nunjucks Environment
var env = nunjucks.configure('app', {watch: false});
env.addFilter('comma', commaFilter);
In your nunjucks template:
{{ number_var|comma }}
MIT © Ryan Murphy