Update a JavaScript function body without overwriting the whole code.

main.js

/*
In the code below, "form_validation" is a JavaScript function provided by a third-party component. This function validates
and then sends the form values. Since these two behaviors are linked, it's impossible to only validate the form in this state.
But we can update the function without overwriting the whole code.
*/

if (typeof form_validation === 'function') {
    var definition = form_validation.toString()
        .replace(/myForm\.submit\(\);?/, 'return true;');

    var functionParameters = definition.substring(definition.indexOf('(') + 1, definition.indexOf(')')).split(',');
    var functionBody = definition.substring(definition.indexOf('{') + 1, definition.lastIndexOf('}'));

    window.form_validation = new Function(functionParameters, functionBody);
}