JavaScript output

JavaScript can "display" data in different ways:

• Writing into an HTML element, using innerHTML.
• Writing into the HTML output using document.write().
• Writing into an alert box, using window.alert().
• Writing into the browser console, using console.log().

Using innerHTML

To access an HTML element, JavaScript can use the document.getElementById(id) method.

The id attribute defines the HTML element. The innerHTML property defines the HTML content:

                  <p id="example-1"></p>
  
                  <script>
                    document.getElementById("example-1").innerHTML = 5 + 6;
                </script>
              

Live preview

Using document.write()

For testing purposes, it is convenient to use document.write():

                <h2>My First Web Page</h2>
                <p>My first paragraph.</p>
    
                <p>Never call document.write after the document has finished loading.
                  It will overwrite the whole document.</p>
    
                <script>
                    document.write(5 + 6);
                </script>
            

Live preview

Note: Be careful. Using document.write() after an HTML document is loaded, will delete all existing HTML:

Using window.alert()

You can use an alert box to display data:

                <button type="button" onclick="window.alert(5 + 6)">Click me to see an alert</button>
            

Live preview

Using console.log()

For debugging purposes, you can use the console.log() method to display data.

Note: You will learn more about debugging in a later chapter.

                  <script>
                    console.log(5 + 6);
                </script>
              

To open the console, click [ctrl] (or [fn]) + [F12] in the browser, and then click "Console".