Question 49590 in Javascript Question posted in
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Whats the difference between predicate and callback function in typescript? – Javascript

Here is my code I am trying to learn.

let depList : Array<number>;
depList = [1,2,3,4,5,6,7,8,9];

let numResults = depList.filter(num => num > 5); // predicate
console.log(numResults);

let resultFind = depList.find(num => num ==5); // predicate
console.log(resultFind);

let sum = depList.reduce((acc, num) => acc + num); // callback function
console.log(sum);

What’s the difference betweeen predicate and callback function here?

I tried going through the method definition but seems hard to understand the differnece.

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2

Answers


  1. 0

    There is a big overlap.

    A "predicate" is a function that takes parameters and returns a boolean. It is always a boolean as it is always related to some logical check. Examples

    const isEmpty = x => x.length === 0;
const isOdd   = n => n % 2 === 1;
const isEven  = n => n % 2 === 0;

    A "callback" is any function that is passed in as a parameter. It might be a predicate (returning boolean) or not.

    In the case of .filter() and .find() the callback they accept is a predicate.

    A predicate needs not be used as a callback. And a callback needs not be a predicate. The two are orthogonal.

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  2. 0

    As indicated, a predicate is a function that takes a single argument, and returns a boolean. A callback is a function that is passed to another function.

    In typescript, predicates are significant because they can be used for type narrowing. For example:

    const myArray:Array<null | number>();
...
const filtered = myArray.filter((n):n is number => n != null)

    Because the predicate was annotated with n is number, it is now a type predicate. This means that filtered now has the type number[] – the type system knows that all the nulls have been filtered from the array.

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