Utilizing Array.prototype.reduce to replicate methods in Array's prototype

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This is an exercise and a challenge to see how far can I go with reduce and how many built-in Array methods I can replicate with it.

Notes

I use ES2015 arrow functions in all the examples below, that's because arrow functions make the code easier to read and in most cases turn the code snippet into a bite-sized one.
The first part of the code is used to generate random values for the reduce function to work with:

var arr = new Array(100)
  .fill('')
  .map(() => Math.floor(Math.random() * 10))

Example

So for example if we want to use concat, we will do the following:

// define the arr array which contains our sample
var arr = new Array(100)
  .fill('')
  .map(() => Math.floor(Math.random() * 10));

// definition the concat function utilizing reduce
var concat = (arr, arr2) =>
  arr2.reduce((memo, current) => {
    return memo.push(current) && memo;
  }, arr);

// and then use concat on arr and another array as the 2nd parameter of concat
concat(arr, [1,2,3]);

	// definition
	var concat = (arr, arr2) =>
	arr2.reduce((memo, current) => {
	return memo.push(current) && memo;
	}, arr);

	// usage
	concat(arr, [1,2,3]);

	// definition
	var every = (arr, check) =>
	arr.reduce((memo, current) => {
	return memo && check(current) ? true : false;
	}, true);

	// usage
	every(arr, (num) => num >= 0);
	every(arr, (num) => num % 2 === 0);

	// definition
	var filter = (arr, check) =>
	arr.reduce((memo, current) => {
	return check(current) ? memo.push(current) && memo : memo;
	}, []);

	// usage
	filter(arr, (num) => num === 5);
	filter(arr, (num) => num % 2 === 0);

	// definition
	var findIndex = (arr, check) =>
	arr.reduce((memo, current, index) => {
	return !memo && check(current) ? index : memo;
	}, undefined);

	// usage
	findIndex(arr, (num) => num === 5);
	findIndex(arr, (num) => num % 2 === 0);

	// definition
	var forEach = (arr, check) =>
	arr.reduce((memo, current, index) => {
	check(current, index);
	});

	// usage
	forEach(arr, (item, index) => console.log(item, index));

	// definition
	var indexOf = (arr, val) =>
	arr.reduce((memo, current, index) => {
	return memo === -1 && val === current ? index : memo;
	}, -1);

	// usage
	indexOf(arr, 5);

	// definition
	var join = (arr, separator) =>
	arr.reduce((memo, current) => {
	return memo === '' ? memo + current : memo + (separator \|\| '') + current;
	}, '');

	// usage
	join(['a', 'b', 3], '.');

	// definition
	var map = (arr, check) =>
	arr.reduce((memo, current) => {
	return memo.push(check(current)) && memo;
	}, []);

	// usage
	map(arr, (item) => item * 5);

	// definition
	var reverse = (arr) =>
	arr.reduce((memo, current) => {
	return memo.unshift(current) && memo;
	}, []);

	// usage
	reverse(arr);

	// definition
	var some = (arr, check) =>
	arr.reduce((memo, current) => {
	return !memo && check(current) ? true : memo;
	}, false);

	// usage
	some(arr);

ToC

Notes

Example