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Parenthesis' hold optional parameters to be passed and received to and by a functions, object methods or object constructors. for example:
<script language="JavaScript"><!-- // first define a simple function: function functionName(a, b) { return a*b; } // invoke the simple function passing two values: var c = functionName(2, 3); //--></script>
Square brackets are used in arrays to access an entry in an array either by its position (index) or by its name (associate arrays), for example:
<script language="JavaScript"><!-- // first define an array using the Array object constructor: var a = new Array(); // populate the first entry in the array: a[0] = 'Hello World'; // populate the array entry named 'x': a['x'] = 'dlroW olleH'; // retrieve the values using the window object's alert() method: alert(a[0]); alert(a['x']); //--></script>