A variable that has not been assigned a value is of type undefined. A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. A function returns undefined if a value was not returned.
undefined
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Note: While you can use undefined as an identifier (variable name) in any scope other than the global scope (because undefined is not a reserved word), doing so is a very bad idea that will make your code difficult to maintain and debug.
You can use undefined and the strict equality and inequality operators to determine whether a variable has a value. In the following code, the variable x is not initialized, and the if statement evaluates to true.
Note: The strict equality operator (as opposed to the standard equality operator) must be used here, because x == undefined also checks whether x is null, while strict equality doesn't. This is because null is not equivalent to undefined.
\n A variable that has not been assigned a value is of type undefined. A\n method or statement also returns undefined if the variable that is being\n evaluated does not have an assigned value. A function returns undefined if\n a value was not returned.\n
\n You can use undefined and the strict equality and inequality operators to\n determine whether a variable has a value. In the following code, the variable\n x is not initialized, and the if statement evaluates to true.\n
\n Note: The strict equality operator (as opposed to the\n standard equality operator) must be used here, because\n x == undefined also checks whether x is null,\n while strict equality doesn't. This is because null is not equivalent to\n undefined.\n
In mathematics, the term undefined is often used to refer to an expression which is not assigned an interpretation or a value (such as an indeterminate form, which has the propensity of assuming different values).[1] The term can take on several different meanings depending on the context. For example:
In ancient times, geometers attempted to define every term. For example, Euclid defined a point as "that which has no part". In modern times, mathematicians recognize that attempting to define every word inevitably leads to circular definitions, and therefore leave some terms (such as "point") undefined (see primitive notion for more).
This more abstract approach allows for fruitful generalizations. In topology, a topological space may be defined as a set of points endowed with certain properties, but in the general setting, the nature of these "points" is left entirely undefined. Likewise, in category theory, a category consists of "objects" and "arrows", which are again primitive, undefined terms. This allows such abstract mathematical theories to be applied to very diverse concrete situations.
The expression .mw-parser-output .sfracwhite-space:nowrap.mw-parser-output .sfrac.tion,.mw-parser-output .sfrac .tiondisplay:inline-block;vertical-align:-0.5em;font-size:85%;text-align:center.mw-parser-output .sfrac .num,.mw-parser-output .sfrac .dendisplay:block;line-height:1em;margin:0 0.1em.mw-parser-output .sfrac .denborder-top:1px solid.mw-parser-output .sr-onlyborder:0;clip:rect(0,0,0,0);height:1px;margin:-1px;overflow:hidden;padding:0;position:absolute;width:1px0/0 is undefined in arithmetic, as explained in division by zero (the same expression is used in calculus to represent an indeterminate form).
The set of numbers for which a function is defined is called the domain of the function. If a number is not in the domain of a function, the function is said to be "undefined" for that number. Two common examples are f ( x ) = 1 x \textstyle f(x)=\frac 1x , which is undefined for x = 0 \displaystyle x=0 , and f ( x ) = x \displaystyle f(x)=\sqrt x , which is undefined (in the real number system) for negative x \displaystyle x .
This is a very useful value: undefined :: a You can put it anywhereand it will compile. When evaluated, it throws an exception"undefined". But thanks to laziness you can put it anywhere you've notwritten yet:
If you just want to get a function to compile, but you've not finishedwriting half of it, just stick an undefined in there. For example, sayI'm writing a function to read from a file, print it out and parse itwith a given parser; I haven't thought of the parse bit yet, but I wantto check the rest of the function's okay, so I just stick an undefinedin there:
Undefined means that a variable has been declared but has not been assigned a value. In JavaScript, properties and functions can only belong to objects. Since undefined is not an object type, calling a function or a property on such a variable causes the TypeError: Cannot read property 'length' of undefined.
Since the variable myVar is declared but not initialized, it is undefined. When it is passed to the myFunc() function, the property length is attempted to be accessed. Since a is undefined at that point, running the code causes the following error:
The compilers are required to issue diagnostic messages (either errors or warnings) for any programs that violates any C syntax rule or semantic constraint, even if its behavior is specified as undefined or implementation-defined or if the compiler provides a language extension that allows it to accept such program. Diagnostics for undefined behavior are not otherwise required.
undefined (value) is a primitive and is the sole value of the Undefined type. Any property that has not been assigned a value, assumes the undefined value. (ECMA 4.3.9 and 4.3.10). A function without a return statement, or a function with an empty return statement returns undefined. The value of an unsupplied function argument is undefined.
Needless to say, re-assigning values to the undefined variable is very bad practice, and in fact its not allowed by ECMA 5 (though amongst the current set of full browser releases, only Safari enforces this).
Yes, generally well understood but worth re-stating: undefined is distinct from null which is also a primitive value representing the intentional absence of a value. The only similarity between undefined and null is they both coerce to false.
System services in ArcGIS Server display as 'undefined' instead of 'Geoprocessing Service' after the service name. This indicates the service is broken. Examples of a broken and working services are presented in the images below:
The variable p points to the string literal "hello!\n", and the two assignments below try to modify that string literal. What does this program do? According to section 2.14.5 paragraph 11 of the C++ standard, it invokes undefined behavior:
I can hear people screaming "But wait, I can compile this no problem and get the output yellow" or "What do you mean undefined, string literals are stored in read-only memory, so the first assignment attempt results in a core dump". This is exactly the problem with undefined behavior. Basically, the standard allows anything to happen once you invoke undefined behavior (even nasal demons). If there is a "correct" behavior according to your mental model of the language, that model is simply wrong; The C++ standard has the only vote, period.
Other examples of undefined behavior include accessing an array beyond its bounds, dereferencing the null pointer, accessing objects after their lifetime ended or writing allegedly clever expressions like i++ + ++i.
Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).
3.4.3 1 undefined behavior behavior, upon use of a nonportable or erroneousprogram construct or of erroneousdata, for which this InternationalStandard imposes no requirements
2NOTE Possible undefined behaviorranges from ignoring the situationcompletely with unpredictable results,to behaving during translation orprogram execution in a documentedmanner characteristic of theenvironment (with or without theissuance of a diagnostic message), toterminating a translation or execution(with the issuance of a diagnosticmessage).
undefined behavior:You are doing something wrong. For example, you have a very large value in an int that doesn't fit in char. How do you put that value in char? actually there is no way! Anything could happen, but the most sensible thing would be to take the first byte of that int and put it in char. It is just wrong to do that to assign the first byte, but thats what happens under the hood.
The language doesn't specify the evaluation, left to right or right to left! So an unspecified behavior may or mayn't result in an undefined behavior, but certainly your program should not produce an unspecified behavior.
IMO far too many "things" are left undefined, unspecified, that's easy to say and even to give examples of, but hard to fix. It should also be noted that it is not all that difficult to avoid most of the problems and produce portable code.
The terms unspecified behavior, undefined behavior, and implementation-defined behavior are used to categorize the result of writing programs whose properties the Standard does not, or cannot, completely describe. The goal of adopting this categorization is to allow a certain variety among implementations which permits quality of implementation to be an active force in the marketplace as well as to allow certain popular extensions, without removing the cachet of conformance to the Standard. Appendix F to the Standard catalogs those behaviors which fall into one of these three categories. 2ff7e9595c
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