This section provides a tutorial example showing that primitive parameters are passed by value and object parameters are passed by reference.
As I mentioned in a previous section, JavaScript documentation says that primitive parameters
are passed by value and object parameters are passed by reference.
I want to test these rules using the following JavaScript tutorial example:
<html>
<!-- Swap_Values_and_Arrays.html
Copyright (c) 2008 by Dr. Herong Yang, http://www.herongyang.com/
-->
<head><title>Swap Values and Arrays</title></head>
<body>
<pre>
<script type="text/javascript">
// Defining a function to swap values
function swap_values(left, right) {
var temp;
temp = left;
left = right;
right = temp;
}
// Defining a function to swap arrays
function swap_arrays(left, right) {
var temp;
for (var i=0; i<left.length; i++) {
temp = left[i];
left[i] = right[i];
right[i] = temp;
}
}
var first = "Black";
var second = "White";
document.write("\nTest 1: Swapping two string variables\n");
document.write(" Before call: "+first+" | "+second+"\n");
swap_values(first,second);
document.write(" After call: "+first+" | "+second+"\n");
var colors = new Array("Red", "Green", "Blue", "Yellow");
var animals = new Array("Dog", "Cat", "Fish", "Bird");
document.write("\nTest 2: Swapping two array objects\n");
document.write(" Before call: "+colors+"\n");
swap_arrays(colors,animals);
document.write(" After call: "+colors+"\n");
</script>
</pre>
</body>
</html>
Here is the output of this tutorial example:
Test 1: Swapping two string variables
Before call: Black | White
After call: Black | White
Test 2: Swapping two array objects
Before call: Red,Green,Blue,Yellow
After call: Dog,Cat,Fish,Bird
The output shows no surprises.
When statement "swap_values(first,second);" is executed,
Copies of values stored in "first" and "second" are passed into the swap_values()
function. The swap operation inside swap_values() is performed on copied values.
Not on original values in "first" and "second".
This is why "first" still contains "Blank" after the function call.
When statement "swap_arrays(colors,animals);" is executed,
References to arrays stored in "colors" and "animals" are passed into the swap_arrays()
function. The swap operation inside swap_arrays() is performed on original arrays.
This is why the "colors" array contains animals now.
