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The query also flags unused imports, functions and classes (which, of course, are just unused variables at the end of the day). This is now made more explicit in the description.
71 lines
2.3 KiB
XML
71 lines
2.3 KiB
XML
<!DOCTYPE qhelp PUBLIC
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"-//Semmle//qhelp//EN"
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"qhelp.dtd">
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<qhelp>
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<overview>
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<p>
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Unused local variables make code hard to read and understand. Any computation used to initialize
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an unused variable is wasted, which may lead to performance problems.
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</p>
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<p>
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Similarly, unused imports and unused functions or classes can be confusing. They may even be
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a symptom of a bug caused, for example, by an incomplete refactoring.
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</p>
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</overview>
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<recommendation>
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<p>Remove the unused program element.</p>
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</recommendation>
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<example>
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<p>
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In this code, the function <code>f</code> initializes a local variable <code>x</code> with a
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call to the function <code>expensiveComputation</code>, but later on this variable is never read.
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Removing <code>x</code> would improve code quality and performance.
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</p>
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<sample src="examples/UnusedVariable.js" />
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<p>
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A slightly subtle case is shown below, where a function expression named <code>f</code> is
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assigned to a variable <code>f</code>:
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</p>
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<sample src="examples/UnusedVariable2.js" />
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<p>
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Note that this example involves two distinct variables, both named <code>f</code>: the global
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variable to which the function is assigned, and the variable implicitly declared by the
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function expression. The call to <code>f()</code> refers to the former variable, whereas the
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latter is unused. Hence the example can be rewritten as follows, eliminating the useless
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variable:
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</p>
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<sample src="examples/UnusedVariable2Good.js" />
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<p>
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A similar situation can occur with ECMAScript 2015 module exports, as shown in the following example:
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</p>
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<sample src="examples/UnusedVariable3.js" />
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<p>
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Again, the named function expression implicitly declares a variable <code>f</code>, but because
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the export statement is a default export, this variable is unused and can be eliminated:
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</p>
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<sample src="examples/UnusedVariable3Good.js" />
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</example>
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<references>
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<li>Coding Horror: <a href="http://blog.codinghorror.com/code-smells/">Code Smells</a>.</li>
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<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en/docs/web/JavaScript/Reference/Operators/function#Named_function_expression">Named function expressions</a>.</li>
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<li>Mozilla Developer Network: <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/export#Using_the_default_export">Using the default export</a>.</li>
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</references>
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</qhelp>
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