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I'm not quite sure what goes on between <script> tags. For example, the following gives a reference error and type error in Chrome:
<html> <head> <script type="text/javascript"> T.prototype.test = function() { document.write("a"); } </script> <script type="text/javascript"> function T() {} var v = new T(); v.test(); </script> </head> <body> </body> </html> But this works:
<html> <head> <script type="text/javascript"> </script> <script type="text/javascript"> T.prototype.test = function() { document.write("a"); } function T() {} var v = new T(); v.test(); </script> </head> <body> </body> </html> 
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An HTML page can contain multiple <script> tags in the <head> or <body> tag. The browser executes all the script tags, starting from the first script tag from the beginning.
Multiple <SCRIPT> Tags Up to this point all of the JavaScript Code was in one <SCRIPT> tag, this does not need to be the case. You can have as many <SCRIPT></SCRIPT> tags as you would like in a document.
In JavaScript, scopes are created by code blocks, functions, modules. While const and let variables are scoped by code blocks, functions or modules, var variables are scoped only by functions or modules. Scopes can be nested. Inside an inner scope you can access the variables of an outer scope.
Learn HTML You can place the <script> tags, containing your JavaScript, anywhere within your web page, but it is normally recommended that you should keep it within the <head> tags. The <script> tag alerts the browser program to start interpreting all the text between these tags as a script.
The upper script is executed first in the first example, so it doesn't know about T yet, hence the error.
In the second example, T is well defined and known anywhere as soon as the script tag is executed. This is due to function declarations being hoisted to the top, no matter what the order is, they are always available. Function declaration hoisting is more deeply explained here
The second example after hoisting is applied:
var v, T = function(){}; /* using comma like this is short-hand for: var v; var T = function(){}; */ T.prototype.test = function() { document.write("a"); }; v = new T(); v.test(); They are each parsed in the global context, but in order. The entire first script tag is parsed and executed before the second one is considered. As part of parsing a script, function declarations are recognized ahead of time, which is why the second one works while the first one doesn't.
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