In Typescript what is the exclamation mark bang operator when dereferencing a member

Question

When looking at the sourcecode for a tslint rule  I came across the following statement   if  node parent  kind     ts SyntaxKind ObjectLiteralExpression        return      Notice the   operator after node parent  Interesting   I first tried compiling the file locally with my currently installed version of TS  1 5 3   The resulting error pointed to the exact location of the bang     tsc --noImplicitAny memberAccessRule ts  noPublicModifierRule ts 57 24   error TS1005      expected    Next I upgraded to the latest TS  2 1 6   which compiled it without issue  So it seems to be feature of TS 2 x  But the transpilation ignored the bang completely  resulting in the following JS   if  node parent kind     ts SyntaxKind ObjectLiteralExpression        return      My Google fu has thus far failed me    What is TS s exclamation mark operator  and how does it work

User · Answer

Louis  answer is great  but I thought I would try to sum it up succinctly   The bang operator tells the compiler to temporarily relax the  not null  constraint that it might otherwise demand  It says to the compiler   As the developer  I know better than you that this variable cannot be null right now

User · Answer

non-null assertion operator With the non-null assertion operator we can tell the compiler explicitly that an expression has value other than null or undefined  This is can be useful when the compiler cannot infer the type with certainty but we more information than the compiler  Example TS code function simpleExample nullableArg  number   undefined   null       const normal  number   nullableArg            Compile err            Type  number   null   undefined  is not assignable to type  number            Type  undefined  is not assignable to type  number   2322      const operatorApplied  number   nullableArg           compiles fine because we tell compiler that null   undefined are excluded     Compiled JS code Note that the JS does not know the concept of the Non-null assertion operator since this is a TS feature  x000D   x000D   use strict   function simpleExample nullableArg        const normal   nullableArg      const operatorApplied   nullableArg     x000D   x000D   x000D

User · Answer

That s the non-null assertion operator  It is a way to tell the compiler  this expression cannot be null or undefined here  so don t complain about the possibility of it being null or undefined   Sometimes the type checker is unable to make that determination itself   It is explained here      A new   post-fix expression operator may be used to assert that its operand is non-null and non-undefined in contexts where the type checker is unable to conclude that fact  Specifically  the operation x  produces a value of the type of x with null and undefined excluded  Similar to type assertions of the forms  lt T gt x and x as T  the   non-null assertion operator is simply removed in the emitted JavaScript code    I find the use of the term  assert  a bit misleading in that explanation  It is  assert  in the sense that the developer is asserting it  not in the sense that a test is going to be performed  The last line indeed indicates that it results in no JavaScript code being emitted

[typescript] In Typescript, what is the ! (exclamation mark / bang) operator when dereferencing a member?

Examples related to typescript

Examples related to tslint