You cannot pass values from javascript to the template that way because the template is going to be rendered before the response goes back to the browser for the javascript engine to evaluate. The only way the template renderer would be able to resolve the value of name specified in the javascript code would be to interpret the string embedded in <script></script>.
Update. Let's look at your second attempt, the one that you say has worked. You have:
<body>
<button onclick="js_fn('{{ py_fn('some_string') }}')">Click me</button>
<script>
function js_fn(variable) {
alert(variable);
}
</script>
</body>
Presumably this is in some partial (say _index.html). A view that has py_fn in scope, loads _index.html, evaluates the string "py_fn('some_string')", and replaces {{ py_fn('some_string') }} with the result of that evaluation. Let's say py_fn is the identity function on strings: it's a unary function that takes a string and immediately returns it. Then, the result of evaluating "py_fn('some_string')" will be the string 'some_string', which will be substituted back, obtaining the final, so-called "rendered" template:
<body>
<button onclick="js_fn('some_string')">Click me</button>
<script>
function js_fn(variable) {
alert(variable);
}
</script>
</body>
This string will be part of the response body of the request, so the browser will dump the button on the window, evaluate the js code inside the script block, which will create a global variable js_fn on the window, which will take something and alert it. The button, when clicked on, will call js_fn with the constant some_string, always. As you can see, there is no passing of values from JS to Python/Flask.
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