Typescript interface default values

Question

I have the following interface in TypeScript   interface IX       a  string      b  any      c  AnotherType     I declare a variable of that type and I initialize all the properties  let x  IX         a   abc       b  null      c  null     Then I assign real values to them in an init function later  x a    xyz  x b   123 x c   new AnotherType     But I don t like having to specify a bunch of default null values for each property when declaring the object when they re going to just be set later to real values  Can I tell the interface to default the properties I don t supply to null  What would let me do this   let x  IX         a   abc      without getting a compiler error  Right now it tells me     TS2322  Type      is not assignable to type    IX    Property  b  is missing in type

User · Answer

It is depends on the case and the usage  Generally  in TypeScript there are no default values for interfaces  If you don t use the default values You can declare x as  let x  IX   undefined     declaration  x   undefined  Then  in your init function you can set real values  x         a   xyz      b  123     c  new AnotherType       In this way  x can be undefined or defined - undefined represents that the object is uninitialized  without set the default values  if they are unnecessary  This is loggically better than define  quot garbage quot   If you want to partially assign the object  You can define the type with optional properties like  interface IX       a  string      b   any      c   AnotherType    In this case you have to set only a  The other types are marked with   which mean that they are optional and have undefined as default value  In any case you can use undefined as a default value  it is just depends on your use case

User · Answer

Default values to an interface are not possible because interfaces only exists at compile time  Alternative solution  You could use a factory method for this which returns an object which implements the XI interface  Example  class AnotherType     interface IX       a  string      b  any      c  AnotherType   null    function makeIX     IX       return       a   abc       b  null      c  null          const x   makeIX     x a    xyz   x b   123  x c   new AnotherType     The only thing I changed with regard to your example is made the property c both AnotherType   null  Which will be necessary to not have any compiler errors  This error was also present in your example were you initialized null to property c

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Can I tell the interface to default the properties I don t supply to null  What would let me do this  No  You cannot provide default values for interfaces or type aliases as they are compile time only and default values need runtime support Alternative But values that are not specified default to undefined in JavaScript runtimes  So you can mark them as optional  interface IX     a  string    b   any    c   AnotherType    And now when you create it you only need to provide a  let x  IX         a   abc      You can provide the values as needed  x a    xyz  x b   123 x c   new AnotherType

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While  Timar s answer works perfectly for null default values  what was asked for   here another easy solution which allows other default values  Define an option interface as well as an according constant containing the defaults  in the constructor use the spread operator to set the options member variable  interface IXOptions       a   string      b   any      c   number    const XDefaults  IXOptions         a   default       b  null      c  1    export class ClassX       private options  IXOptions       constructor XOptions  IXOptions            this options        XDefaults     XOptions               public printOptions    void           console log this options a           console log this options b           console log this options c             Now you can use the class like this   const x   new ClassX   a   set      x printOptions      Output   set null 1

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You can use the Partial mapped type as explained in the documentation  https   www typescriptlang org docs handbook release-notes typescript-2-1 html  In your example  you ll have    interface IX       a  string      b  any      c  AnotherType     let x  Partial lt IX gt          a   abc

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It s best practice in case you have many parameters to let the user insert only few parameters and not in specific order  For example  bad practice  foo a  b  c  d  e   Good practice  foo  d 3    The way to do it is through interfaces  You need to define the parameter as an interface like  interface Arguments       a       b        c       d       e      And define the function like  foo arguments  Arguments   Now interfaces variables can t get default values  so how do we define default values  Simple  we define default value for the whole interface  foo           a          b 1          c 99          d 88          e                            Arguments   Now if the user pass  foo  d 3    The actual parameters will be        a      b 1      c 99      d 3      e                        I understood it from the following link so big credit    https   medium com better-programming named-parameters-in-typescript-e32c763d2b2e

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I stumbled on this while looking for a better way than what I had arrived at  Having read the answers and trying them out I thought it was worth posting what I was doing as the other answers didn t feel as succinct for me  It was important for me to only have to write a short amount of code each time I set up a new interface  I settled on     Using a custom generic deepCopy function   deepCopy    lt T extends    gt  input  any   T   gt      return JSON parse JSON stringify input         Define your interface  interface IX       a  string      b  any      c  AnotherType          and define the defaults in a separate const   const XDef   IX         a          b  null      c  null       Then init like this   let x   IX   deepCopy XDef     That s all that s needed       however     If you want to custom initialise any root element you can modify the deepCopy function to accept custom default values  The function becomes   deepCopyAssign    lt T extends    gt  input  any  rootOverwrites   any   T   gt      return JSON parse JSON stringify      input     rootOverwrites           Which can then be called like this instead   let x   IX   deepCopyAssign XDef    a  customInitValue         Any other preferred way of deep copy would work  If only a shallow copy is needed then Object assign would suffice  forgoing the need for the utility deepCopy or deepCopyAssign function   let x   IX   object assign     XDef    a  customInitValue        Known Issues   It will not deep assign in this guise but it s not too difficult to modify deepCopyAssign to iterate and check types before assigning  Functions and references will be lost by the parse stringify process  I don t need those for my task and neither did the OP  Custom init values are not hinted by the IDE or type checked when executed

User · Answer

You could use two separate configs  One as the input with optional properties  that will have default values   and another with only the required properties  This can be made convenient with  amp  and Required  interface DefaultedFuncConfig     b   boolean     interface MandatoryFuncConfig     a  boolean     export type FuncConfig   MandatoryFuncConfig  amp  DefaultedFuncConfig    export const func    config  FuncConfig   Required lt FuncConfig gt    gt       b  true       config         will compile func   a  true     func   a  true  b  true         will error func   b  true     func

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You can t set default values in an interface  but you can accomplish what you want to do by using Optional Properties  compare paragraph  3    https   www typescriptlang org docs handbook interfaces html  Simply change the interface to   interface IX       a  string      b   any      c   AnotherType     You can then do   let x  IX         a   abc      And use your init function to assign default values to x b and x c if those properies are not set

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You can implement the interface with a class  then you can deal with initializing the members in the constructor   class IXClass implements IX       a  string      b  any      c  AnotherType       constructor obj  IX       constructor a  string  b  any  c  AnotherType       constructor             if  arguments length    1                this a   arguments 0  a              this b   arguments 0  b              this c   arguments 0  c            else               this a   arguments 0               this b   arguments 1               this c   arguments 2                       Another approach is to use a factory function   function ixFactory a  string  b  any  c  AnotherType   IX       return           a  a          b  b          c  c           Then you can simply   var ix  IX   null       ix   new IXClass          or ix   ixFactory

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My solution   I have created wrapper over Object assign to fix typing issues   export function assign lt T gt     args  T     Partial lt T gt      T     return Object assign apply Object          args        Usage   env base ts  export interface EnvironmentValues   export interface EnvironmentValues     isBrowser  boolean    apiURL  string     export const enviromentBaseValues  Partial lt EnvironmentValues gt        isBrowser  typeof window      undefined       export default enviromentBaseValues    env dev ts  import   EnvironmentValues  enviromentBaseValues   from    env base   import   assign   from     utilities    export const enviromentDevValues  EnvironmentValues   assign lt EnvironmentValues gt           apiURL    api          enviromentBaseValues     export default enviromentDevValues

[typescript] Typescript interface default values

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