Please indicate the type of object to be used with lodash's reduce

Question

Please indicate the type of object to be used with lodash's reduce

When using Lodash's _.reduce() with an object, I encountered a TypeScript error (as shown below) that indicates it expects an array instead. How can I resolve the type mismatch in this scenario?

interface Fees {
    CardHandlingFee: number;
    AirlineSurcharge: number;
}

const fees: Fees = {
    CardHandlingFee: 2,
    AirlineSurcharge: 3
};

let total = 100;

// Argument of type 'Fees' is not assignable to parameter of type 'NumericDictionary'.
// Index signature is missing in type 'Fees'.
total += _.reduce(fees, (sum: number, v: number) => sum + v, 0);

typescript lodash typescript-typings

Answer 1

Answer №1

Unfortunately, specifying the type of fees as Fees changes it from being treated as an Object, which would have passed the check for NumericDictionary<T> due to TypeScript's structural typing.

Here are two possible solutions:

1) Remove the type declaration from the fees variable. TypeScript can infer the type and when you pass the object where a Fees instance is required, it will work because of structural typing.

interface Fees {
    CardHandlingFee: number;
    AirlineSurcharge: number;
}

const fees = {
    CardHandlingFee: 2,
    AirlineSurcharge: 3
};

let total = 100;
total += _.reduce(fees, (sum, v) => sum + v, 0);

2) Define Fees as an extension to NumericDictionary<number>

interface Fees extends _.NumericDictionary<number> {
    CardHandlingFee: number;
    AirlineSurcharge: number;
}

const fees: Fees = {
    CardHandlingFee: 2,
    AirlineSurcharge: 3
};

let total = 100;    
total += _.reduce(fees, (sum, v) => sum + v, 0);

Also, there's no need to declare the types of sum and v in the reduce function as they will be inferred from the type of fees.

Answer 2

Unfortunately, specifying the type of fees as Fees changes it from being treated as an Object, which would have passed the check for NumericDictionary<T> due to TypeScript's structural typing.

Here are two possible solutions:

1) Remove the type declaration from the fees variable. TypeScript can infer the type and when you pass the object where a Fees instance is required, it will work because of structural typing.

interface Fees {
    CardHandlingFee: number;
    AirlineSurcharge: number;
}

const fees = {
    CardHandlingFee: 2,
    AirlineSurcharge: 3
};

let total = 100;
total += _.reduce(fees, (sum, v) => sum + v, 0);

2) Define Fees as an extension to NumericDictionary<number>

interface Fees extends _.NumericDictionary<number> {
    CardHandlingFee: number;
    AirlineSurcharge: number;
}

const fees: Fees = {
    CardHandlingFee: 2,
    AirlineSurcharge: 3
};

let total = 100;    
total += _.reduce(fees, (sum, v) => sum + v, 0);

Also, there's no need to declare the types of sum and v in the reduce function as they will be inferred from the type of fees.

Please indicate the type of object to be used with lodash's reduce

Answer №1

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