What is the question mark for in a Typescript parameter name

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export class Thread     id  string    lastMessage  Message    name  string    avatarSrc  string     constructor id   string                name   string                avatarSrc   string        this id   id    uuid        this name   name      this avatarSrc   avatarSrc          In id  what s the   for

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This is to make the variable of Optional type  Otherwise declared variables shows  undefined  if this variable is not used   export interface ISearchResult       title  string      listTitle string    entityName   string    lookupName  string    lookupId  string

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It is to mark the parameter as optional    TypeScript handbook https   www typescriptlang org docs handbook functions html optional-and-default-parameters TypeScript Deep Dive https   basarat gitbook io typescript type-system functions optional-parameters

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The   in the parameters is to denote an optional parameter  The Typescript compiler does not require this parameter to be filled in  See the code example below for more details     baz  number   undefined means  the second argument baz can be a number or undefined       undefined  is default parameter syntax      if the parameter is not filled in it will default to undefined     Although default JS behaviour is to set every non filled in argument to undefined     we need this default argument so that the typescript compiler    doesn t require the second argument to be filled in function fn1  bar  string  baz  number   undefined   undefined           do stuff       All the above code can be simplified using the   operator after the parameter    In other words fn1 and fn2 are equivalent in behaviour function fn2  bar  string  baz   number           do stuff      fn2  foo   3      works fn2  foo       works  fn2       Compile time error  Expected 1-2 arguments  but got 0    An argument for  bar  was not provided    fn1  foo   3      works fn1  foo       works  fn1       Compile time error  Expected 1-2 arguments  but got 0    An argument for  bar  was not provided

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parameter   type is a shorthand for parameter  type   undefined

[typescript] What is the question mark for in a Typescript parameter name

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