Update HTML blog pages

computer-science-blog/pages/js.html

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+<!-- Reading School 2018, HTML page by Matthew Grove, Year 10 -->
+<h2>JavaScript</h2>
+<p>
+    JavaScript (JS) is a high-level, client-side language, meaning that it is like a human language
+    (rather than, for example, binary) and runs on the end user's computer. It is used for many purposes,
+    including editing content on a webpage, performing mathematical operations and storing variables.
+    Each line of JavaScript should end in a semicolon, for example:
+    <br>
+    <code>
+        // this code sets the content of the element with the ID "id_name" to "Hello World!"<br>
+        document.getElementById("id_name").innerHTML = "Hello World!";
+    </code>
+    <br>
+    A single line comment in JS is denoted by a double forward slash (<code>//</code>); a multi line comment
+    is started with <code>/*</code> and ended with <code>*/</code>.
+</p>