Custom Operators

In some cases you may want to create your own custom operator, by reusing an existing one, or by combining several existing operators. And here we show you how to do just that.

See also: iter-ops-extras - collection of complete operators.

Using a single operator

When using a single existing operator, you can just forward into it:

import {aggregate, Operation} from 'iter-ops';

// joins strings with separator:
function join<T>(separator?: string): Operation<T, string> {
    return aggregate(a => a.join(separator));
}

Usage example:

const i = pipe('word', join(','));

console.log(...i); //=> w,o,r,d

Using several operators

When using several existing operators, you need to return a callback with chain:

import {distinct, map, Operation} from 'iter-ops';

// selects unique key-values, then remaps into just values:
export function uniqueValues<T, R>(keySelector: (value: T, index: number) => R): Operation<T, R> {
    return i => {
        const k = distinct(keySelector)(i); // process + chain to the source iterator
        return map(keySelector)(k); // process + chain to the previous iterator
    }
}

Usage example:

const input: { value: number }[] = [{value: 1}, {value: 2}, {value: 3}, {value: 2}];

const i = pipe(input, uniqueValues(a => a.value)); //=> pick unique object-values, and return just values

console.log(...i); //=> 1 2 3