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| Property/method value type: | Boolean primitive | ||||||||
| JavaScript syntax: | - | anOperand1 && anOperand2 | |||||||
| Argument list: | anOperand1 | A Boolean value | |||||||
| anOperand2 | Another Boolean value | ||||||||
Traditionally in programming environments, the logical AND operator yields true only when both operands are true. However, the specifics of this are slightly different in JavaScript, and although the results may appear to be functionally the same, there is a subtle but important difference.
First, lets deal with the normal and expected behavior of a Logical AND operator. The truth table shows the result of this operator for two Boolean primitive values:
Now, the implementation is expected to conform to the ECMA standard. This sets out the following method of evaluation for a Logical AND operator:
Evaluate and convert the first operand using the ToBoolean() method.
If it is false, then evaluate and return the second operand.
Otherwise return the evaluation of the first operand.
To all intents and purposes the external perceived behavior is the same because another ToBoolean() conversion is likely to take place in the context that the expression is used - an if() statement or an outer logical expression for example.
The associativity is from left to right.
Refer to the operator precedence topic for details of execution order.
The result is the Boolean value true if both operands are true, otherwise Boolean false is returned.
<HTML>
<HEAD></HEAD>
<BODY>
<TABLE BORDER=1 CELLPADDING=2>
<TR>
<TH>A</TH>
<TH>B</TH>
<TH>AND</TH>
</TR>
<SCRIPT>
for(a=0; a<2; a++)
{
for(b=0; b<2; b++)
{
document.write("<TR ALIGN=CENTER><TD>");
document.write(Boolean(a));
document.write("</TD><TD>");
document.write(Boolean(b));
document.write("</TD><TD>");
document.write(Boolean(a && b));
document.write("</TD></TR>");
}
}
</SCRIPT>
</TABLE>
</BODY>
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JavaScript Programmer's Reference, Cliff Wootton Wrox Press (www.wrox.com) Join the Wrox JavaScript forum at p2p.wrox.com Please report problems to support@wrox.com © 2001 Wrox Press. All Rights Reserved. Terms and conditions. | ||
