Script tags can appear in either
the or the of the document. If there??™s a link to an external JavaScript file, it loads
that link before continuing to evaluate the page. Embedding third-party scripts can lead to
apparent slow page load times if the remote server is overburdened and can??™t return the file
quickly enough. It??™s usually best to load those scripts as close to the bottom of the HTML page
as possible.
My PageScripts that you build should appear at the head of the document and need to be loaded
as soon as possible because they??™ll probably include functionality that the rest of the page
relies on.
CHAPTER 1 n THE STATE OF JAVASCRIPT 4
Code Evaluation
Code evaluation is the process by which the browser takes the code you??™ve written and turns
it into executable code. The first thing it will do is test to see whether the code is syntactically
correct. If it isn??™t, it will fail right off the bat. If you try and run a function that has a syntax
error (for example, a missing bracket somewhere), you??™ll likely receive an error message saying
that the function is undefined.
After the browser has ensured that the code is valid, it evaluates all the variables and functions
within the script block. If you have to call a function that??™s in another script block or in
another file, be sure that it has loaded before the current script element is loaded.
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