Can a predicate be an object in JavaScript?

Question

Can a predicate be an object in JavaScript (as in C++)?

For example, is it possible to find the index of an element in an array (with arr.findIndex(pred)) with a predicate like this?

    class Predicate
    {
        constructor()
        {
            this.sum = 0;
        }

        evaluate(elem)
        {
            this.sum += elem;
            return sum > 25;
        }
    }

    const pred = new Predicate();

    const index = arr.findIndex(pred);

EDIT1: If no, what is the easiest way to find the index of an element that makes the sum of it and all the previous elements exceed 25?

Can a predicate be an object in JavaScript? -> NO – Anand Undavia Nov 15 '18 at 13:14 — Anand Undavia, Nov 15 '18 at 13:14

Abhi · Accepted Answer · 2018-11-15T13:59:39.050

1

It should be a callback, this the function definition

arr.findIndex(callback(element[, index[, array]])[, thisArg])

You can provide your Predicate objects evaluate method. Like this

const index = arr.findIndex(pred.evaluate,pred);// pass pred as thisArg

edited Nov 15 '18 at 13:59

answered Nov 15 '18 at 13:16

Abhi

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1

You need to either bind the method or pass the instance as a `thisArg` – Yury Tarabanko Nov 15 '18 at 13:19
findIndex is being called on arr (Array), method is bound to the array. It runs perfectly fine. – Abhi Nov 15 '18 at 13:31
my bad, i removed his logic and just tested the callback. Edited answer.Thank you for pointing out! http://jsfiddle.net/4ga3sLy9/3/ – Abhi Nov 15 '18 at 13:59

score 1 · Answer 2 · answered Nov 15 '18 at 13:23

Predicate has to be a function. MDN. But you could always create a function that returns another function.

const runningSum = (sum, current = 0) => value => (current += value) > sum

const index = [1, 2, 3, 4, 5, 6, 7, 8, 9].findIndex(runningSum(25))

console.log(index)

score 1 · Answer 3 · answered Nov 15 '18 at 13:25

There is no such thing as an evaluate method (that would get invoked when an object gets called like a function) in JavaScript. However, functions themselves are objects and you could even create them with class syntax. It's not very useful though, in JS you would just write a closure to create a function with instance-specific values:

function predicate(max) {
    var sum = 0;
    return function(elem) {
        sum += elem;
        return sum > max;
    };
}

const pred = predicate(25);

const index = arr.findIndex(pred);

Can a predicate be an object in JavaScript?

3 Answers3