I want to to use Mystring as alert's parameter so when mouse on the link, it will alert Mystring?
<html>
<head>
<script>
window.onload = initAll;
function initAll() {
var Mystring = "Hello world";
var aTag = document.getElementsByTagName('a')[0];
aTag.onmouseover = runAlert;
}
function runAlert() {
alert(Mystring);
}
</script>
</head>
<body>
<a href="#">Over me</a>
</body>
</html>
Your Mystring variable is scoped inside initAll function. It is not available outside that function. What you might want to do is define it outside and assign the value in initAll, like this:
window.onload = initAll;
var myString;
function runAlert() {
alert(myString);
}
function initAll() {
myString = "Hello world";
var aTag = document.getElementsByTagName('a')[0];
aTag.onmouseover = runAlert;
}
If this all you are going to do, you can define an in-line anonymous function, like this
aTag.onmouseover = function() {
alert(myString);
};
Note: Normally JavaScript variables are named with camelCase convention.
Note 1: It is always a good practice to define the called functions before the calling functions, like I have shown in my answer.
If you want to keep your scopes and avoid global pollution, you can assign a parameter to your alert function:
function initAll() {
var Mystring = "Hello world";
var aTag = document.getElementsByTagName('a')[0];
aTag.onmouseover = function(){runAlert(Mystring)};
}
function runAlert(str) {
alert(str);
}
See here: http://jsfiddle.net/VZ69x/
runAlert function on init and assign its result to the onmouseover method. It won't work as you expected it, as you'd get the alert when the page loads. This way, onmouseover is a function that calls another function.aTag.onmouseover = runAlert(Mystring); would be the same as aTag.onmouseover = undefined; and onmouseover should be a function. See this simple example I made for you: jsfiddle.net/LL3Hq