Cannot invoke an expression whose type lacks a call signature

Question

I have apple and pears - both have an isDecayed attribute   interface Apple       color  string      isDecayed  boolean     interface Pear       weight  number      isDecayed  boolean      And both types can be in my fruit basket  multiple times    interface FruitBasket      apples  Apple       pears  Pear        Let s assume for now my basket is empty   const fruitBasket  FruitBasket     apples      pears          Now we take randomly one kind out of the basket   const key  keyof FruitBasket   Math random    gt  0 5    apples    pears    const fruits   fruitBasket key     And of course nobody likes decayed fruits so we pick only the fresh ones   const freshFruits   fruits filter  fruit    gt   fruit isDecayed     Unfortunately Typescript tells me      Cannot invoke an expression whose type lacks a call signature  Type    callbackfn   value  Apple  index  number  array  Apple       any  thisArg   any     Apple           has no compatible call signatures    What s wrong here - is it just that Typescript doesn t like fresh fruits or is this a Typescript bug   You can try it yourself in the official Typescript Repl

User · Answer

I had the same issue with numeral  a JS library  The fix was to install the typings again with this command   npm install --save  types numeral

User · Answer

Perhaps create a shared Fruit interface that provides isDecayed  fruits is now of type Fruit   so the type can be explicit  Like this   interface Fruit       isDecayed  boolean     interface Apple extends Fruit       color  string     interface Pear extends Fruit       weight  number     interface FruitBasket       apples  Apple        pears  Pear        const fruitBasket  FruitBasket     apples      pears        const key  keyof FruitBasket   Math random    gt  0 5    apples    pears    const fruits  Fruit     fruitBasket key    const freshFruits   fruits filter  fruit    gt   fruit isDecayed

User · Answer

TypeScript supports structural typing  also called duck typing   meaning that types are compatible when they share the same members  Your problem is that Apple and Pear don t share all their members  which means that they are not compatible  They are however compatible to another type that has only the isDecayed  boolean member  Because of structural typing  you don  need to inherit Apple and Pear from such an interface   There are different ways to assign such a compatible type   Assign type during variable declaration  This statement is implicitly typed to Apple     Pear     const fruits   fruitBasket key     You can simply use a compatible type explicitly in in your variable declaration   const fruits    isDecayed  boolean       fruitBasket key     For additional reusability  you can also define the type first and then use it in your declaration  note that the Apple and Pear interfaces don t need to be changed    type Fruit     isDecayed  boolean    const fruits  Fruit     fruitBasket key     Cast to compatible type for the operation  The problem with the given solution is that it changes the type of the fruits variable  This might not be what you want  To avoid this  you can narrow the array down to a compatible type before the operation and then set the type back to the same type as fruits   const fruits  fruitBasket key   const freshFruits    fruits as   isDecayed  boolean      filter fruit   gt   fruit isDecayed  as typeof fruits    Or with the reusable Fruit type   type Fruit     isDecayed  boolean    const fruits  fruitBasket key   const freshFruits    fruits as Fruit    filter fruit   gt   fruit isDecayed  as typeof fruits    The advantage of this solution is that both  fruits and freshFruits will be of type Apple     Pear

User · Answer

As mentioned in the github issue originally linked by  peter in the comments   const freshFruits    fruits as  Apple   Pear     filter  fruit   Apple   Pear     gt   fruit isDecayed

[javascript] Cannot invoke an expression whose type lacks a call signature

Examples related to javascript

Examples related to typescript

Examples related to types