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Improved checking of nullable operands in expressions #13483

merged 6 commits into from Jan 17, 2017


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commented Jan 14, 2017

This PR improves our checking of nullable operands in expressions. Specifically, we now error in the following cases:

  • If either operand of a + operator is nullable, and neither operand is of type any or string.
  • If either operand of a -, *, **, /, %, <<, >>, >>>, &, |, or ^ operator is nullable.
  • If either operand of a <, >, <=, >=, or in operator is nullable.
  • If the right operand of an instanceof operator is nullable.
  • If the operand of a +, -, ~, ++, or -- unary operator is nullable.

An operand is considered nullable if the type of the operand is null or undefined or a union type that includes null or undefined. Note that the union type case only only occurs in --strictNullChecks mode because null and undefined disappear from unions in classic type checking mode.

Fixes #12795.

!!! error TS2365: Operator '+' cannot be applied to types 'boolean' and 'boolean'.
!!! error TS2531: Object is possibly 'null'.

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DanielRosenwasser Jan 14, 2017


Object is definitely null, and typically we don't think of objects as being operands to arithmetic operations (whereas we do when it comes to primitives). Would be nice if we could be more precise on the error message.

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ahejlsberg Jan 14, 2017

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Two orthogonal issues:

First, we could do extra work to omit the word possibly when the type is exactly null or undefined. However, it is very rare for someone to explicitly use null or undefined as an operand, so it's not clear that it really matters all that much. This is the type of error that only surfaces in a test.

Second, we could consider using the word value instead of object when reporting for expression operands--but it's really the same error that we use for where obj may be null or undefined. The downside would be error message searchability, but since the error is pretty obvious that may not be an issue.

@ahejlsberg ahejlsberg merged commit d6fde0c into master Jan 17, 2017

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@ahejlsberg ahejlsberg deleted the operatorsAndNullableTypes branch Jan 17, 2017

@TimvdLippe TimvdLippe referenced this pull request Feb 6, 2017


Fix several compilation errors with Typescript 2.2 #444

1 of 1 task complete

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