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An undefined variable in the source code of a computer program is a variable that is accessed in the code but has not been declared by that code. [1] In some programming languages, an implicit declaration is provided the first time such a variable is encountered at compile time. In other languages such a usage is considered to be sufficiently ...
Lacking the distinct features of "usual" JavaScript, obfuscation techniques like JSFuck can assist malicious JavaScript code in bypassing intrusion prevention systems [17] or content filters. For instance, the lack of alphanumeric characters in JSFuck and a flawed content filter allowed sellers to embed arbitrary JSFuck scripts in their eBay ...
An undefined value must not be confused with empty string, Boolean "false" or other "empty" (but defined) values. Depending on circumstances, evaluation to an undefined value may lead to exception or undefined behaviour, but in some programming languages undefined values can occur during a normal, predictable course of program execution.
The PHP processor only parses code within its delimiters. Anything outside its delimiters is sent directly to the output and not parsed by PHP. The only open/close delimiters allowed by PSR-1 [6] are "<?php" and "?>" or <? = and ?>. The purpose of the delimiting tags is to separate PHP code from non-PHP data (mainly HTML).
The final value of k is undefined. The answer that it must be 10 assumes that it started at zero, which may or may not be true. Note that in the example, the variable i is initialized to zero by the first clause of the for statement. Another example can be when dealing with structs.
A snippet of JavaScript code with keywords highlighted in different colors. The syntax of JavaScript is the set of rules that define a correctly structured JavaScript program. The examples below make use of the log function of the console object present in most browsers for standard text output.
However, more often it is taken to refer to the actual use of an entity before any declaration; that is, the first reference to second in the code above is a forward reference. [2] [3] Thus, we may say that because forward declarations are mandatory in Pascal, forward references are prohibited. An example of (valid) forward reference in C++:
It is the responsibility of the programmer to write code that never invokes undefined behavior, although compiler implementations are allowed to issue diagnostics when this happens. Compilers nowadays have flags that enable such diagnostics, for example, -fsanitize=undefined enables the "undefined behavior sanitizer" in gcc 4.9 [2] and in clang ...