How to declare a type as nullable in TypeScript

Question

I have an interface in TypeScript   interface Employee      id  number      name  string      salary  number      I would like to make salary as a nullable field  Like we can do in C    Is this possible to do in TypeScript

User · Answer

You can just implement a user-defined type like the following   type Nullable lt T gt    T   undefined   null   var foo  Nullable lt number gt    10     ok var bar  Nullable lt number gt    true     type  true  is not assignable to type  Nullable lt number gt   var baz  Nullable lt number gt    null     ok  var arr1  Nullable lt Array lt number gt  gt     1 2      ok var obj  Nullable lt Object gt           ok      Type  number    is not assignable to type  string          Type  number  is not assignable to type  string  var arr2  Nullable lt Array lt string gt  gt     1 2

User · Answer

Union type is in my mind best option in this case   interface Employee     id  number     name  string     salary  number   null        Both cases are valid let employe1  Employee     id  1  name   John   salary  100    let employe2  Employee     id  1  name   John   salary  null      EDIT   For this to work as expected  you should enable the strictNullChecks in tsconfig

User · Answer

i had this same question a while back   all types in ts are nullable  because void is a subtype of all types  unlike  for example  scala    see if this flowchart helps - https   github com bcherny language-types-comparison typescript

User · Answer

Nullable type can invoke runtime error  So I think it s good to use a compiler option --strictNullChecks and declare number   null as type  also in case of nested function  although input type is null  compiler can not know what it could break  so I recommend use   exclamination mark    function broken name  string   null   string     function postfix epithet  string        return name charAt 0        the     epithet     error   name  is possibly null       name   name     Bob     return postfix  great       function fixed name  string   null   string     function postfix epithet  string        return name  charAt 0        the     epithet     ok       name   name     Bob     return postfix  great        Reference  https   www typescriptlang org docs handbook advanced-types html type-guards-and-type-assertions

User · Answer

Just add a question mark   to the optional field   interface Employee     id  number     name  string     salary   number

User · Answer

All fields in JavaScript  and in TypeScript  can have the value null or undefined   You can make the field optional which is different from nullable   interface Employee1       name  string      salary  number     var a  Employee1     name   Bob   salary  40000       OK var b  Employee1     name   Bob        Not OK  you must have  salary  var c  Employee1     name   Bob   salary  undefined       OK var d  Employee1     name  null  salary  undefined       OK     OK class SomeEmployeeA implements Employee1       public name    Bob       public salary   40000        Not OK  Must have  salary  class SomeEmployeeB implements Employee1       public name  string      Compare with   interface Employee2       name  string      salary   number     var a  Employee2     name   Bob   salary  40000       OK var b  Employee2     name   Bob        OK var c  Employee2     name   Bob   salary  undefined       OK var d  Employee2     name  null  salary   bob        Not OK  salary must be a number     OK  but doesn t make too much sense class SomeEmployeeA implements Employee2       public name    Bob

User · Answer

To be more C  like  define the Nullable type like this   type Nullable lt T gt    T   null   interface Employee     id  number     name  string     salary  Nullable lt number gt       Bonus   To make Nullable behave like a built in Typescript type  define it in a global d ts definition file in the root source folder  This path worked for me   src global d ts

User · Answer

type MyProps       workoutType  string   null

[typescript] How to declare a type as nullable in TypeScript?

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