Read a variable name in Javascript from Flask

Question

I am passing variable from Flask to JavaScript using the method render_template.

I followed this post: Passing variables from flask to javascript, but it is not working for me

Here is my code:

@app.route('/')
def index():
    return render_template('index.html')

@app.route('/debug')
def index_debug():
    return render_template('index.html', debug=True)

I would like to read it from a JavaScript file. I tried using this method:

this is my html file:

<head> 
    <script type="text/javascript">
        var debugMode = {{ debug }};
    </script>
    <script type="text/javascript" src="https://code.jquery.com/jquery-3.4.1.js" crossorigin="anonymous"></script>
    <script src="/static/index.js"></script>
</head>

But when i try accessing this variable in my index.js. the variable debugMode is undefined.

Maciej Trzciński · Accepted Answer · 2020-02-20 08:37:54Z

0

Try this

<head> 
 <script type="text/javascript">
    window.debugMode = {{ debug }};
 </script>
 <script src="/static/index.js"></script>
</head>

If you want to read a variable from HTML in JS you ought to declare this var in window scope and read in JS file like this.

const myNewVar = window.debugMode

answered Feb 20, 2020 at 8:37

Maciej Trzciński

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6 Comments

Kenny Smith Over a year ago

Thanks for the response. But on my JS file, I am trying to read it : let debugMode= window.debugMode But window.debugMode is undefined

Maciej Trzciński Over a year ago

When you console.log window.debugMode in Chrome console, it's also undefined?

Kenny Smith Over a year ago

Yes. I tired to print it also in the console

Maciej Trzciński Over a year ago

Did you also check the page source? To be sure that window.debug has any value.

Kenny Smith Over a year ago

Yes. This is the header file in the HTML: <head> <script type="text/javascript"> window.debugMode = True; </script> <script src="/static/index.js"></script> <link rel="stylesheet" href="/static/index.css"> <script type="text/javascript" src="code.jquery.com/jquery-3.4.1.js" crossorigin="anonymous"></script> <script src="/static/index.js"></script> </head> (I can't put code or image in the comment)

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