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JavaScript Programmer’s Reference Warnings: . Be careful not to miss one of the equals signs when testing for equality. You can accidentally assign to an LValue and not realize it. See also: = (Assign), Associativity, Equal to (==), Equality expression, Identically equal to (===), NOT Equal to (!=), NOT Identically equal to (!==), Operator, Operator Precedence, Relational operator Cross-references: ECMA 262 edition 2 section 11.9 ECMA 262 edition 3 section 11.9 Error (Definition) That which happens when your script fails to execute properly. Availability: ECMAScript edition 2 The ECMA standard dictates that a compliant implementation will detect and alert on all errors in the code even if the error is in a section of code that is not executed. Such dead code might be in a conditional block for which the condition could never be true. Nevertheless, a compile time warning should be generated. This means that you should not rely on placing code within an if(false) block to prevent its execution. Instead, you should comment out the code to achieve the same effect. This behavior is generally implementation dependent due to the different ways that compilers and interpreters may be designed and built. It also changes as browser versions evolve and of course the kind of error that is thrown can also affect when and how it is detected and managed. See also: Diagnostic message, Error handler, Mathematics Cross-references: ECMA 262 edition 2 section 16 ECMA 262 edition 3 section 16 Error events (Definition) A class of events that are triggered by errors. HTML syntax:
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E Equality operator (Definition) If the native types of the two operands are not the same, then the values are not equal unless a numeric coercion yields equal values from both sides or the operands both yield null/undefined values. If the type of the left operand is undefined or null, it is assumed to be equal to the right operand. If either of the operands is NaN,then they are assumed to be not equal. Positive and negative zero are equal values. Boolean values must be identical values to be considered equal. Two strings of the same length containing the same character sequence (identical copies of one another in terms of Unicode character code points) are assumed to be equal. References to the same object test as equal. References to two objects containing the same property values, even though they may be copies of one another, are not equal. Comparing different types of operands can use coercion techniques to force the comparison to be conducted according to string, numeric or Boolean rules. If one of the operands is an object and the other is not, then the object is converted to a primitive. String comparisons can be by concatenating values of other types to an empty string. It may help to use the parentheses to guarantee that precedence is established as you intend it to be. Here is a forced string comparison: (”" + a) == (”" + b) Numeric comparisons can be forced by subtracting zero. Again, grouping operators help to establish the desired precedence: (a 0) == (b 0) Boolean comparisons can be forced by performing a logical NOT on both operands, in which case precedence control with grouping operators may not be as necessary as it is with the addition and concatenation operators: !a == !b The values null and undefinedare generally considered to be equal although they are distinctly different values. This is because early implementations did not support an explicit undefined value and allowed for it to have the same meaning as the null value. Comparing strings is done simply according to the Unicode character code point values and does not take into account any of the more subtle semantic meanings of those characters as defined in the Unicode version 2.0 specification. Refer to the identity operators for a more exact comparison taking data type into account.
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JavaScript Programmer’s Reference may be more reliable in some implementations than: if(a != null){ someCode} Example code: // Force a string comparison myResult = (a+'’ == b+'’); alert(myResult); // Force a numeric comparison myResult = (a-0 == b-0); alert(myResult); // Force a boolean comparison myResult = (!a == !b); See also: Equal to (==), Equality operator, Expression, Identically equal to (===), NOT Equal to (!=), NOT Identically equal to (!==), Relational expression, Type conversion Cross-references: ECMA 262 edition 2 section 11.9 ECMA 262 edition 3 section 11.9 Wrox Instant JavaScript page 39 Equality operator (Definition) An operator that tests for equality or not. Availability: Property/method value type: ECMAScript edition 2 Boolean primitive There are two equality operators: . The == operator tests for equality. . The != operator tests for inequality. Equality operators deal exclusively with the test for the operands being equal to one another. They yield a true or false result and are generally considered as part of the relational operator set since they are most often used in the same circumstances. Testing two operands for equality follows these basic rules:
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E Equality expression (Definition) Tests for equality require further deductive reasoning on the part of the interpreter. The values are converted to their preferred types. If the types are the same, then the values can be compared easily either as Numbers or Strings. If the types are different, the further conversion is necessary before the comparison can be completed. In that case, Boolean become Numbers as do any other nonnumeric values and numeric comparison predominates. Comparing null with undefined values does not require any conversion and they will compare equal. Comparing values with nullcan expose some bugs in earlier implementations. It may be safer to rely on Boolean conversions and simply test for trueor false. The ECMA standard (edition 3) sets out the rules for testing two values for equality (somewhat simplified): . If they are of different types, they are unequal return false. . If the type of the first argument is undefined or null return true. . For numeric values NaN in either case return false. . Positive and negative zeros are represented differently internally but are equal. . Otherwise equal numbers return true and unequal numbers return false. . For strings, the two values must contain the same sequence of characters for them to be equal. . Boolean values must be equal and they have no special states to consider. . Object references must refer to the same object instance. . The values null and undefined are considered to be equal. . If necessary, ToNumber() and ToPrimitive()functions are called to coerce objects when one value is a primitive and the other is an object. . If all of the above fail to match equal then a false value is returned. It seems odd to assume equality to be true if the type of the arguments is not absolutely clear, but this may be necessary to allow implementations some flexibility in the internal representation of values which may not have a defined type. It might also be strange to see that if either value is NaN then a false value is returned. This is necessary because NaNis indeterminate. It is not a specific value and can be caused by a variety of circumstances when an expression yields an out of bounds value. There is no guarantee that two individual NaN values resulted from the same circumstance and hence they are assumed to be unequal. Referring to the same object also includes joined objects which are an internal mechanism for sharing functionality between objects. This is not exposed to the script interface. Warnings: . Earlier versions of MSIE and Netscape exhibited bugs in the comparison logic between the results of comparisons involving NaN, null and 0, where type conversions led to inconsistent behavior. . Since historically there are known bugs in comparisons involving null values, you should avoid using them. The following: if(a){ someCode}
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JavaScript Programmer’s Reference Example code: // Fuzzy matching of numeric values function almost_equal(aValue1, aValue2) { var myPrecision = 1e-10; if((Math.abs(aValue1 aValue2)) < myPrecision) { return(true); } return(false); } See also: ASCII, Type conversion, typeof, Unicode Cross-references: ECMA 262 edition 2 section 11.9.1 ECMA 262 edition 3 section 11.9.1 Wrox Instant JavaScript page 36 Equality expression (Definition) An expression that tests for equality or not. Availability: Property/method value type: ECMAScript edition 2 Boolean primitive Equality expressions are a special case of Relational expressions. They deal strictly with equality or non-equality. There are two equality operators that you can use to make an equality expression: . The == operator tests for equality. . The != operator tests for inequality. As a general rule, equality expressions will yield a true or false result in a more forgiving way than relational expressions. Passing NaN, undefined and null values to relational expressions may yield undefined values as results, where an equality expression would still return a Boolean value. The comparisons between objects are likely to be a shallow comparison. If you are comparing two objects of the same type, the comparison logic will check to see if you are referring to the same instance. That is a test for identity and not equality. A deeper comparison might compare two similar objects on a property by property basis. They wouldn't be identical, but they may be equivalent. You could simulate this with a script function that returns trueor falsehaving done a deep comparison.
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E Equal to (==) (Operator/equality) Equal to (==) (Operator/equality) Compares two operands for equality. Availability: ECMAScript edition 2 JavaScript 1.0 JScript 1.0 Internet Explorer 3.02 Netscape 2.0 Netscape Enterprise Server 2.0 Opera 3.0 Property/method value type: Boolean primitive JavaScript syntax: -anOperand1 == anOperand2 anOperand1 A value that can reasonably be compared Argument list: anOperand2 A value that can reasonably be compared with anOperand1 The result is the Boolean value trueif anOperand1is numerically or lexically equal to anOperand2, otherwise false is returned. The equality operator is not always transitive. For example, two string objects may represent the same string value. Comparing the objects for equality will yield false because the references to the objects are being compared and not the object values themselves. However, forcing a string comparison may in fact yield a truevalue when testing for equality. Do this by converting the objects as part of the comparison process by type conversion or valueOf() methods. Numeric values may require rounding to take place and testing for equality may be accurate down to a precision finer than your script cares about. You can set a precision value in a variable, then subtract the value you are comparing with from the absolute value of the comparee. If the difference is smaller than your precision value, then the two values are close enough to yield a match. The associativity is left to right. Refer to the operator precedence topic for details of execution order. Refer to the Equality expression topic for a discussion on the ECMA standard definition of the equality testing rules. Warnings: . Be careful not to confuse the single equals with the double equals. Placing a single equals in place of a test for equality will assign the right hand value to the left hand operand and clobber the value accidentally. It will also force the relational expression to yield true as well. The interpreter may be forgiving enough that a run-time error isn’t generated, but the side effects could be subtle and make it hard to diagnose the cause. . A triple equals sign further complicates things as it is a test for identical type as well as equal value.
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JavaScript Programmer’s Reference Enumerator.moveNext() (Method) Moves the enumerator to the next item in the collection. Availability: JScript 3.0 Internet Explorer 4.0 JavaScript syntax: IE myEnumerator.MoveNext() This indexes the enumeratoronwards to the next item in the collection that has not yet been visited. Environment (Definition) The environment is the computing context in which the script is executed. There are a variety of different environments in which a script may be executed. At the time of writing, a JavaScript script could be operating in any of these distinctly different environments: . A Netscape web browser . A MSIE browser . Several other new web browsers . A server CGI environment . A desktop application environment . A UNIX shell . A WebTV set top box . A Liberate TV Navigator set top box . A WAP/WScript mobile phone . An Adobe PDF file reader . An embedded web browser built-into consumer products Each of these has certain advantages and constraints. Most offer special facilities native and unique to that hosting environment. In general, you should be able to determine which of these environments you are operating in. However, there is no standardized way to detect this at present. There may be range limits on values in certain environments and certainly there will be ‘bugs’ in the implementations that are platform specific. It is also very likely that functionality will be more or less incomplete in some environments mostly depending on the maturity of the implementation. See also: Character display semantics, Character set, Execution environment, Host environment, Limits, Script termination Cross-references: Wrox Instant JavaScript page 5
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E Enumerator.item() (Method) You can use this as one way of creating Enumeratorobjects, although it is more popular to use the new Enumerator() technique. This property is useful if you have an object that you want to clone, but you don’t know what sort of object it is. Simply access the constructor belonging to the object you have a reference to. Netscape provides constructors for many objects, virtually all of them in fact, even when it is highly inappropriate to do so. MSIE is far more selective and there are some occasions when you might wish for a constructor that MSIE does not make available. Warnings: . On the Macintosh version of MSIE 5, this property yields a reference to the Date() constructor object. This is obviously a bug and renders the Enumerator unusable on this platform. Enumerator.item() (Method) A reference to the current item in the collection. This method returns the object from the collection that the enumerator is currently accessing. JScript 3.0 User defined myEnumerator.item(anIndex) IE myEnumerator.item(aSelector) IE myEnumerator.item(aSelector, anIndex) anIndex IE Availability: Internet Explorer 4.0 Property/method value type: JavaScript syntax: A zero based index into the collection Argument list: aSelector A textual value that selects all matching objects Refer to: Collection.Item() Enumerator.moveFirst() (Method) Resets the enumerator to point at the first item in the collection. JScript 3.0 myEnumerator.MoveFirst() Availability: Internet Explorer 4.0 JavaScript syntax: IE This relocates the enumerator so that it accesses the first item in the collection.
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JavaScript Programmer’s Reference Method JavaScript JScript N IE Opera Notes atEnd() 3.0 + 4.0 + - item() 3.0 + 4.0 + - moveFirst() 3.0 + 4.0 + - moveNext() 3.0 + 4.0 + - Enumerator() (Constructor) A constructor function for creating new Enumerator objects. Availability: JScript 3.0 Internet Explorer 4.0 Property/method value type: Enumerator object IE new Enumerator() JavaScript syntax: IE new Enumerator(aCollection) Argument list: aCollection A collection of objects to be enumerated This is the constructor function for creating new enumerator objects. Use it with the new operator to manufacture an Enumeratorand then store the reference to it in a variable. Enumerator.atEnd() (Method) A method that returns a flag indicating the end of the collection. Availability: JScript 3.0 Internet Explorer 4.0 Property/method value type: Boolean primitive JavaScript syntax: IE myEnumerator.AtEnd() In your enumeration loop, you can test this method and exit the loop if it returns the Boolean true value. Enumerator.constructor (Property) A reference to the constructorobject for the Enumerator. Availability: JScript 3.0 Internet Explorer 4.0 Property/method value type: Enumerator object JavaScript syntax: IE myEnumerator.constructor You can access the constructor for an existing Enumerator object here.
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E Enumerator object (Object/JScript) The available set of methods and properties is somewhat limited compared with enumerator objects in other languages. Because this is only available on MSIE and is severely dysfunctional on the Macintosh version of MSIE 5, its use is somewhat limited from the portability point of view. It is recommended that you avoid using it for the time being. Later, when it is more widely and reliably available, it may be more useful. Do not confuse DOM NodeList arrays with Enumerator or Collection objects. The NodeListitem() method is subtly different to the Enumerator.Item() method. Warnings: . When tested on MSIE 5 for Macintosh, this object exhibited some very odd behavior. . When passed a FormArray, it complained that it was not a Collection object. When passed a Collection object (document.all) it still complained. However, when passed an Array object, it was happy to accept it. It would allow a new Enumerator to be created with no argument being passed to its constructor function. . When examined, its constructor reported that it was a reference to a Date object. . The object may only be usable on MSIE on the Windows platform until a later version of MSIE supports a corrected implementation. . The naming convention for methods and properties of this object are capitalized in a very untypical way and you need to be aware of this in case you have trouble getting your enumerator to work properly. Example code: // Instantiate a file system object myFileSystem = new ActiveXObject(”Scripting.FileSystemObject”); // Create an enumerator myEnum = new Enumerator(myFileSystem.Drives); // Traverse the Drives collection via the enumerator for(; !myEnum.atEnd(); myEnum.moveNext()) { processDrive(myEnum.item()); } // A function to do something with each disk drive function processDrive(aDrive){…} See also: Collection object, Files object, NodeList object Property JavaScript JScript N IE Opera Notes constructor 3.0 + 4.0 + Warning
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