JS can "display" data in different ways:

(1)Writing into an alert box, using window.alert();

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My first example.
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(2)Writing into the HTML output using document.write();

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But, using document.write() after an HTML document is fully loaded, will delete all existing HTML:

My First Web Page
My first paragraph.
Try it

(3)Writing into an HTML element, using innerHTML;

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My First Web Page
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My First Paragraph.
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(4)Writing into the browser console, using console.log().

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My First Web Page
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My first paragraph.
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 9 Activate debugging in your browser (Chrome, IE, Firefox) with F12, and select "Console" in the debugger menu.10 
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这样就可以按F12调试自己的代码了。