Setting default value for TypeScript object passed as argument

Question

function sayName params   firstName  string  lastName   string         params lastName   params lastName     smith        lt  lt -- any better alternative to this      var name   params firstName   params lastName     alert name      sayName  firstName   bob       I had imagined something like this might work   function sayName params   firstName  string  lastName  string    smith        Obviously if these were plain  arguments you could do it with   function sayName firstName  string  lastName    smith         var name   firstName   lastName      alert name      sayName  bob      And in coffeescript you have access to the conditional existence operator so can do   param lastName     smith    Which compiles to the javascript   if  param lastName    null        param lastName    smith

User · Answer

Without destructuring  you can create a defaults params and pass it in  interface Name      firstName  string     lastName  string     export const defaultName extends Omit lt Name   firstName  gt        lastName   Smith     sayName      defaultName  firstName   Bob

User · Answer

This can be a nice way to do it that does not involve long constructors  class Person       firstName   string    Bob       lastName   string    Smith           Pass in this class as the required params     constructor params  Person               object assign will overwrite defaults if params exist         Object assign this  params              you can still use the typing  function sayName params  Person        let name   params firstName   params lastName     alert name        you do have to call new but for my use case this felt better sayName new Person  firstName   Gordon     sayName new Person  lastName   Thomas

User · Answer

Here is something to try  using interface and destructuring with default values  Please note that  lastName  is optional   interface IName     firstName  string   lastName   string    function sayName params  IName      const   firstName  lastName    Smith      params   const fullName      firstName    lastName      console log  FullName- gt     fullName     sayName   firstName   Bob

User · Answer

Actually  there appears to now be a simple way  The following code works in TypeScript 1 5   function sayName   first  last    Smith      first  string  last   string     void     const name   first         last    console log name      sayName   first   Bob        The trick is to first put in brackets what keys you want to pick from the argument object  with key value for any defaults  Follow that with the   and a type declaration   This is a little different than what you were trying to do  because instead of having an intact params object  you have instead have dereferenced variables   If you want to make it optional to pass anything to the function  add a   for all keys in the type  and add a default of     after the type declaration   function sayName  first  Bob  last  Smith     first   string  last   string           var name   first         last      alert name      sayName

User · Answer

No  TypeScript doesn t have a natural way of setting defaults for properties of an object defined like that where one has a default and the other does not  You could define a richer structure   class Name       constructor public first   string           public last  string    Smith                And use that in place of the inline type definition   function sayName name  Name        alert name first         name last       You can t do something like this unfortunately   function sayName name     first  string  last  string              and then assign a default object matching the signature                 first  null  last   Smith             As it would only set the default if name was undefined

User · Answer

Object destructuring the parameter object is what many of the answers above are aiming for and Typescript now has the methods in place to make it much easier to read and intuitively understand   Destructuring Basics  By destructuring an object  you can choose properties from an object by key name  You can define as few or as many of the properties you like  and default values are set by a basic syntax of let  key   default    object   let  firstName  lastName    Smith     myParamsObject     Compiles to  var firstName   myParamsObject firstName    a   myParamsObject lastName   lastName    a     void 0    Smith     a    Writing an interface  type or class for the parameter object improves legibility     x000D   x000D  type FullName     x000D    firstName  string  x000D      x000D         default  Smith     x000D    lastName     string  x000D    x000D   x000D  function sayName params  FullName    x000D   x000D       Set defaults for parameter object x000D    var   firstName  lastName    Smith     params  x000D   x000D       Do Stuff x000D    var name   firstName         lastName  x000D    alert name   x000D    x000D   x000D     Use it x000D  sayName   x000D    firstName   Bob  x000D      x000D   x000D   x000D

User · Answer

Typescript supports default parameters now   https   www typescriptlang org docs handbook functions html  Also  adding a default value allows you to omit the type declaration  because it can be inferred from the default value   function sayName firstName  string  lastName    Smith       const name   firstName         lastName    alert name      sayName  Bob

[typescript] Setting default value for TypeScript object passed as argument

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