How to Fix JavaScript SyntaxErrors When Parsing Jinja-Rendered JSON Data?

JavaScript SyntaxError with Jinja-Rendered Data

Problem: When attempting to parse JSON data rendered in a Jinja HTML template using JavaScript, a SyntaxError is encountered, indicating an unexpected '&' token.

Cause: Flask's Jinja environment automatically escapes data rendered in HTML templates for security reasons. This is problematic when passing JSON objects to JavaScript, as the escaped characters interfere with parsing.

Solution: Use Flask's tojson Filter or Markup to Mark Data as Safe

To resolve this issue, utilize Flask's tojson filter, which dumps data into JSON and marks it safe for rendering within JavaScript.

return render_template("template.html", data=tree|tojson)

Alternative Approaches:

• Safe Filter (deprecated): This filter, though deprecated, marks data as safe without JSON conversion.

var data = {{ tree|safe }};

• Markup Wrapper: Wrapping the JSON string in Markup before rendering is equivalent to the safe filter.

return render_template("template.html", data=Markup(json.dumps(tree)))

Direct Python Data Usage:

If the rendered data is not intended for JavaScript, consider passing the Python data directly without using JSON conversion or filters.

return render_template("template.html", data=tree)

Implementation:

In the Jinja template:

var data = {{ data }};

for (i in obj) {
   document.write(obj[i].text + "<br />");
}

Alternatively, Jinja data can be utilized directly without JavaScript parsing:

{% for item in tree %}
    <li>{{ item }}</li>
{% endfor %}

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