Is there a way to limit the keys of T to only number fields, where T[keyof T] is a number

I'm looking to restrict the field parameter within this function:

function calculate<T>(source: T[], field: keyof T) {
   for(const item of source) {

   }
}

The goal is to ensure that item[field] will always be a number.

Is there a way to achieve this?

UPDATE:

In addition, how can I limit the non-generic function parameter like so:

function bar(field: keyof MyObject){ }

to prevent situations like

interface MyObject { name: string, value: number; }
bar("name"); // should result in a compiler error
typescript

Answer №1

If you want to restrict the property value of field to only numbers, you can utilize a Record type as shown below:

function calculateSum<T extends Record<K, number>, K extends string>(items: T[], field: K) {
   let sum = 0;
   for(const item of items) {
       const val = item[field]; 
       sum += val;
   }
   return sum;
}

Test it out:

calculateSum([{foo: "adsf", numField: 3}, {numField: 10}], "numField") // Result should be 13
calculateSum([{numField: 3}, {numField: "duuhh"}], "numField") // This will throw an error

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