content/browser/debugger/manual_tests/debugger-watch-expressions.html

<p>Test for watched expression</p>

<p>To begin test, open DevTools, go the Scripts Panel 
and then click this link: <a href="javascript:runTest()">[begin test]</a>.

<p>Perform the following steps, and note the expected results:
    
<ol>
    
<li><p>After clicking the link above, you should now be paused in the body of 
the test method, thanks to the <code>debugger</code> statement.

<li><p>Add the following expressions to the "Watch Expressions" section of the
Scripts panel sidebar pane: "<code>this</code>", "<code>a</code>", 
"<code>b</code>", "<code>c</code>" and "<code>d</code>". Do <b>NOT</b> enter the quotes.

<li><p>The values of the expressions as shown in the window should be 
<code>Object</code> for <code>this</code>, <code>undefined</code> for
the <code>a</code>, <code>b</code>, and <code>c</code> variables, and a 
value of <code>ReferenceError: d is not defined</code>
for the <code>d</code> variable.

<li><p>Note that the value for <code>d</code> should not change for the life of 
the test, as the variable <code>d</code> is never introduced in the program.

<li><p>Step through the code, and you'll see the values of <code>a</code>,
<code>b</code>, and <code>c</code> change, as the variables are assigned.
Also note that as the scope changes due to the function invocation, values
will be changed to refer to their current scope.  The <code>this</code>
expression will change when the method is invoked on the object constructed by
the test.

<li><p>Click different stack frames in the Call Stack section to ensure the
expressions change value appropriately as the current stack frame changes.

</ol>

<script>
function runTest() {
    
    // a nested function
    function subFunction() {
        debugger;
        var a = "a in subFunction()";
        
        subSubFunction();
        
        // another nested function
        function subSubFunction() {
            debugger;
            var b = "b in subSubFunction()";
        }
    }
    
    // a class
    function aClass() {
        this.x = "xxx";
        this.y = "yyy";
    }
    
    aClass.prototype.aMethod = function() {
        debugger;
        var c = "c in aMethod()";
    }
    
    // main logic
    debugger;
    
    var a = "a in runTest()";
    var b = "b in runTest()";
    var c = "c in runTest()";
    
    subFunction();
    
    var object = new aClass();
    object.aMethod();
    
}
</script>