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68 lines
3.0 KiB
XML
68 lines
3.0 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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The TypeScript compiler has to choose which specific overload is called
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when a method with multiple overloads is called.
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The compiler will always choose the textually first overload that does
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not give rise to any type errors with the arguments provided at the
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function call.
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</p>
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<p>
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This behavior can be unintuitive for programmers unfamiliar with the
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type system in TypeScript, and can in some instances lead to situations
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where a programmer writes an overloaded method where only the first
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overload can ever be used.
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</p>
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</overview>
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<recommendation>
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<p>
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Either reorder the method overloads if an overload with more type
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parameters is placed before a similar overload with fewer parameters.
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Alternatively, collapse multiple overloads with identical parameter types by
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creating a single overload that returns a union of the return types
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from the multiple overloads.
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</p>
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</recommendation>
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<example>
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<p>
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In the example below, a programmer has tried to express that a method
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can return multiple possible values by creating multiple overloads
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with identical parameter types. However, only the first overload
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will ever be selected by the TypeScript compiler.
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</p>
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<sample src="examples/UnreachableMethodOverloads.ts" />
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<p>
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The error can be fixed by merging the overloads into a single method
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signature that returns a union of the previous return types.
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</p>
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<sample src="examples/UnreachableMethodOverloadsGood.ts" />
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<p>
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In the example below, an interface <code>Foo</code> declares a method
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<code>create()</code> with two overloads. The only difference between
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the two overloads is the type parameter <code>T</code> in the first
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overload. The TypeScript compiler will always use the first overload
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when <code>create()</code> is called, as a default type will be used
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for the type parameter <code>T</code> if none is provided.
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This default type is <code>unknown</code> in TypeScript 3.5+, and
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<code>{}</code> in earlier versions.
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</p>
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<sample src="examples/UnreachableMethodOverloadsTypeParameters.ts" />
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<p>
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In this example, the error has been fixed by switching the order of the two
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overloads. In this fixed version, if the <code>create()</code> method
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is called with an explicit type argument the second overload will be
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used, as the first overload would give rise to a type error.
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</p>
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<sample src="examples/UnreachableMethodOverloadsTypeParametersGood.ts" />
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</example>
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<references>
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<li>TypeScript specification: <a href="https://github.com/microsoft/TypeScript/blob/30cb20434a6b117e007a4959b2a7c16489f86069/doc/spec-ARCHIVED.md#4.15.1">Overload Resolution</a></li>
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</references>
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</qhelp>
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