Calling a Javascript function from a html file

Question

My javascript and html files named contents.js and page.html:

function sayHello() {
   alert("Hello World")
}

<!DOCTYPE html>
<html>
   <head>     
   </head>
   
   <body>
   <script type="text/javascript" src="contents.js" ></script>
   <script>
      window.onload = sayHello();
   </script>
      
   </body>
</html>

When run the sayHello function isn't being called. In firefox's console it returns the errors:

-SyntaxError: expected expression, got '<' contents.js:1

ReferenceError: sayHello is not defined

Both files are saved in the same folder. And i'm using a node.js express project in eclipse to create the server:

var http = require('http'),
    fs = require('fs');


fs.readFile('./page.html', function (err, html) {
    if (err) {
        throw err; 
    }       
    http.createServer(function(request, response) {  
        response.writeHeader(200, {"Content-Type": "text/html"});  
        response.write(html);  
        response.end();  
    }).listen(8888);
});

Why am I not able to call the 'sayHello' function in the contents.js file from the page.html file?

window.onload = sayHello(); assigns the return value from sayHello to window.onload. What do you return? — Teemu
– Teemu, Commented Aug 4, 2016 at 16:23
You forgot to use a trigger event. Like onload or onpageshow. — Rafael Passarela
– Rafael Passarela, Commented Aug 4, 2016 at 16:26

T.J. Crowder · Accepted Answer · 2016-08-04 16:25:17Z

2

The code

window.onload = sayHello();

...calls sayHello and assigns its return value to window.onload, exactly the way x = foo() calls foo and assigns its return value to x.

To just assign the function reference, don't call it (remove the ()):

window.onload = sayHello;

Side note: The window load event happens very late in the page load cycle, only after all other resources are loaded, including all images. Most of the time, you want to run your code earlier. If so, just put it in a script tag that you include at the end of the HTML, just before the closing </body> tag.

answered Aug 4, 2016 at 16:25

community wiki

T.J. Crowder

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1 Comment

Nathan K Over a year ago

To expand, sayHello() returns undefined so window.onload is being set to undefined which is why nothing happens.

Ayan · Accepted Answer · 2016-08-04 16:30:54Z

1

Executing sayHello() , you arenot returning anystuff so it was not executed. Had sayHello been returning a function it would have invoked that onload.

var sayHello = (function() {
  return function {
    alert("Hello World")
  };
})();

In this situation, one might use sayHello(), as its returning a function.

window.onload = sayHello;

function sayHello() {
  alert("Hello World")
}

<!DOCTYPE html>
<html>

<head>
</head>

<body>
  <script type="text/javascript" src="contents.js"></script>

</body>


</html>

edited Aug 4, 2016 at 16:30

answered Aug 4, 2016 at 16:23

Ayan

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8 Comments

Andrew Li Over a year ago

Please add an explanation. Just code doesn't help anyone.

Ayan Over a year ago

Hey @AndrewL. Thanks for pointing it out, I just about to update it. I hope you would reconsider your opinion. Will be catious next time bro!

user2562530 Over a year ago

I tried this and the reference error is now gone but im still getting: SyntaxError: expected expression, got '<' contents.js:1 and the alert is not appearing.

Ayan Over a year ago

Hey @user2562530 Can you just run the snippet I shared, it has the alert(). just run it once.

user2562530 Over a year ago

Okay it works here but when I copy it to eclipse i still get the syntax error. What can I do?

|

Robmeister2015 · Accepted Answer · 2016-08-04 16:24:10Z

-1

Put it inside the body tag like this.

<body onload="sayHello()"></body>

answered Aug 4, 2016 at 16:24

Robmeister2015

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