let say I have two arrays of typed object (same type), and I want to merge them, checking if an object already exists, then sum the old and new amount and return a new array with updated values

model:

class Ingredient {
  constructor(public name: string, public amount: number){ }
}

arrays:

  private ingredients: Ingredient[] = [
    new Ingredient('farina', 500),
    new Ingredient('burro', 80),
    new Ingredient('uccellini', 5)
  ];

  private newIngredients: Ingredient[] = [
    new Ingredient('uova', 5),
    new Ingredient('pancetta', 80),
    new Ingredient('uccellini', 8)
  ];

when I try to create a method to check and merge arrays, I have an ERROR logged before to start writing my code!:

addIngredients(newIngredients: Ingredient[]) {

  this.ingredients
    .concat(this.newIngredients)
    .reduce((result, curr) => {

    });
}

This is the error:

error TS2345: Argument of type '(result: Ingredient, curr: Ingredient) => 
void' is not assignable to parameter of type 
'(previousValue: Ingredient, currentValue: Ingredient, currentIndex: number, array: Ingredient[]) ...'.
Type 'void' is not assignable to type 'Ingredient'.

I cannot move on, please help me!

let say I have two arrays of typed object (same type), and I want to merge them, checking if an object already exists, then sum the old and new amount and return a new array with updated values

model:

class Ingredient {
  constructor(public name: string, public amount: number){ }
}

arrays:

  private ingredients: Ingredient[] = [
    new Ingredient('farina', 500),
    new Ingredient('burro', 80),
    new Ingredient('uccellini', 5)
  ];

  private newIngredients: Ingredient[] = [
    new Ingredient('uova', 5),
    new Ingredient('pancetta', 80),
    new Ingredient('uccellini', 8)
  ];

when I try to create a method to check and merge arrays, I have an ERROR logged before to start writing my code!:

addIngredients(newIngredients: Ingredient[]) {

  this.ingredients
    .concat(this.newIngredients)
    .reduce((result, curr) => {

    });
}

This is the error:

error TS2345: Argument of type '(result: Ingredient, curr: Ingredient) => 
void' is not assignable to parameter of type 
'(previousValue: Ingredient, currentValue: Ingredient, currentIndex: number, array: Ingredient[]) ...'.
Type 'void' is not assignable to type 'Ingredient'.

I cannot move on, please help me!

• javascript
• arrays
• angular
• typescript
• reduce

• 2 You should return an Ingredient inside the function passed to reduce. Same type as result. Also ask yourself why you are reducing the array there? It seems highly unlikely that a method with the intention of adding something to an array would also reduce it to a single element. – Balázs Édes Commented Mar 30, 2018 at 23:04
• try to replace private ingredients: Ingredient[] with private ingredients: any – Radonirina Maminiaina Commented Mar 30, 2018 at 23:04
• 2 reduce returns "void" when it has no inner code block hence the error, i would suggest also setting a default value for the accumulator of the reduce function which is the second argument. I.E array#reduce((a,c) => a+c, 0) where 0 is the default value for accumulator – Shanon Jackson Commented Mar 30, 2018 at 23:06
• @BalázsÉdes thanks, the error disappeared when return result is provided. Do you think I'm in a wrong way to do the job? I want an array with the matching elements are reduced by the amount – ufollettu Commented Mar 30, 2018 at 23:12
• 2 @ufollettu Read the docs and examples on Array.prototype.reduce, I think you expect it to do something else – Balázs Édes Commented Mar 31, 2018 at 12:58

1 Answer 1

In your .reduce method, return result and the error will disappear.