Introduction
In this article I wanted to share with you with the most popular JavaScript methods but implemented by scratch.
A little understanding of how JavaScript methods works inside will always help you as a developer.
So these are the most popular methods in JavaScript implemented from scratch. Check this out if you are interested how JavaScript methods works inside. This is also a light version of how this work, the real method specified in ECMAScript is much more complex.
Methods mentioned on this article are:
map, reduce, filter, sort, every, includes, slice, splice, shift, indexOf.
1. map()
const fakeMap = function (callback) {
const newArray = [];
// 'this' refers to the array
for (let i = 0; i < this.length; i++) {
newArray[i] = callback(this[i], i);
}
return newArray;
};
Array.prototype.fakeMap = fakeMap;
[1, 2, 3].fakeMap((n) => n + 1); // output [2, 3, 4]
2. reduce()
const fakeReduce = function (callback, accumulator) {
for (let i = 0; i < this.length; i++) {
accumulator = callback(accumulator, this[i]);
}
return accumulator;
};
Array.prototype.fakeReduce = fakeReduce;
const reducer = (previousValue, currentValue) => previousValue + currentValue;
[1, 2, 3].fakeReduce(reducer, 0); // output 6
3. filter()
const fakeFilter = function (callback) {
const newArray = [];
for (let i = 0; i < this.length; i++) {
if (callback(this[i])) {
newArray.push(this[i]);
}
}
return newArray;
};
Array.prototype.fakeFilter = fakeFilter;
[1, 2, 3, 4, 5].fakeFilter((n) => n > 2); // output [3, 4, 5]
4. sort()
const fakeSort = function (callback) {
const newArray = [...this];
for (let i = 0; i < newArray.length; i++) {
for (let j = 0; j < newArray.length - 1; j++) {
if (callback(newArray[j], newArray[j + 1]) > 0) {
const temp = newArray[j + 1];
newArray[j + 1] = newArray[j];
newArray[j] = temp;
}
}
}
// array is sorted
return newArray;
};
Array.prototype.fakeSort = fakeSort;
[3, 5, 1, 4, 2].fakeSort((a, b) => a - b); // output [1, 2, 3, 4, 5]
5. every()
const fakeEvery = function (callback) {
for (let i = 0; i < this.length; i++) {
if (!callback(this[i])) {
return false;
}
}
return true;
};
Array.prototype.fakeEvery = fakeEvery;
[1, 2, 3, 4, 5].fakeEvery((n) => n > 3); // output false
6. includes()
const fakeIncludes = function (item) {
for (let i = 0; i < this.length; i++) {
if (this[i] === item) {
return true;
}
}
return false;
};
Array.prototype.fakeIncludes = fakeIncludes;
[1, 2, 3, 4, 5].fakeIncludes(5); // output true
7. slice()
const fakeSlice = function (start, end) {
let newArray = [];
if (!end) {
for (let i = 0; i < this.length; i++) {
if (i >= start) {
newArray.push(this[i]);
}
}
return newArray;
}
for (let i = 0; i < this.length; i++) {
if (i >= start && i <= end) {
newArray.push(this[i]);
}
}
return newArray;
};
Array.prototype.fakeSlice = fakeSlice;
[1, 2, 3, 4, 5].fakeSlice(2, 3); // output [3, 4]
8. splice()
const fakeSplice = function (start, deleteCount, ...items) {
let newArray = [];
// if we only provide start fakeSplice(start)
if (!deleteCount) {
for (let i = 0; i < this.length; i++) {
if (i < start) {
newArray.push(this[i]);
}
}
this.length = 0;
this.push.apply(this, newArray);
return;
}
// if we only provide start and deleteCount fakeSplice(start, deleteCount)
if (!items) {
for (let i = 0; i < this.length; i++) {
if (i < start || i >= start + deleteCount) {
newArray.push(this[i]);
}
}
this.length = 0;
this.push.apply(this, newArray);
return;
}
// if we provide all arguments to the function
for (let i = 0; i < this.length; i++) {
if (i === start + deleteCount) {
newArray = [...newArray, ...items];
}
if (i < start || i >= start + deleteCount) {
newArray.push(this[i]);
}
}
this.length = 0;
this.push.apply(this, newArray);
return;
};
Array.prototype.fakeSplice = fakeSplice;
const planets = [
"Mercury",
"Venus",
"Earth",
"Mars",
"Jupiter",
"Saturn",
"Uranus",
"Neptune",
];
planets.fakeSplice(2, 2, "Pluto");
console.log(planets); // output ["Mercury", "Venus", "Pluto", "Jupiter", "Saturn", "Uranus", "Neptune"]
9. shift()
const fakeShift = function () {
let newArray = [];
for (let i = 1; i < this.length; i++) {
newArray.push(this[i]);
}
this.length = 0;
this.push.apply(this, newArray);
};
Array.prototype.fakeShift = fakeShift;
const arr = [1, 2, 3, 4, 5];
arr.fakeShift();
console.log(arr); // output [2, 3, 4, 5]
10. indexOf()
const fakeIndexOf = function (item) {
for (let i = 0; i < this.length; i++) {
if (item === this[i]) {
return i;
}
}
return -1;
};
Array.prototype.fakeIndexOf = fakeIndexOf;
[1, 2, 3, 4, 5].fakeIndexOf(4); // output 3
Conclusion
We just implemented the most used methods in JavaScript and get a better understanding of how they work so we can use them more effectively.
Thank you for your time!
Oldest comments (16)
sortshould work in place:tc39.es/ecma262/#sec-array.prototy...
developer.mozilla.org/en-US/docs/W...
Hi!
These are only a light versions of popular methods in JS, and of course the original methods mentioned in ECMAScript are much more complex and useful.
Thank you for comment :)
Yes, I totally agree with you.
You don't have to know how car engine works inside to drive a car, what I meant was
knowing how car engine works, makes you a better understanding of a car itself.
And you may be surprised but assembly language is still in use, so knowing such stuff definitely makes you a better dev 😜.
Cheers!
These are only a light versions of "the engine". So we won't get a full understanding of it, but a light one.
For those who want to familiarize with the internals of JS methods e.g.
Array.prototype.mapyou can find this information here: developer.mozilla.org/en-US/docs/W...Thanks 😀
Nice Post
Good article, but you forgot to include the
return -1for.indexOfThanks for noticing! I made an update.
Useful
It's not the same thing as rebuilding a car engine, but it shows he knows javascript and datastructure. Sometimes you need an original algorithm if you software is a bit innovative, if you just do lame integration work then it's ok you don't need to understand much.
I don't say ALL integration work are lame, I say that there are integration work that are lame that's why they are automatable why do you think lowcode/nocode are now rising and usable by just business people ;) dev.to/lepinekong/comment/1l7ab
I do SystemThinking : things are not separate ;) If I recruit a coder I will indeed want to be sure he master the fundamentals if not I would rather recruit the guy/girl for doing lowcode or nocode.
Nice. Fundamentals are worth knowing. Developers are like engineers who build engines, so if you don’t understand how a single part of the engine you want to build works, than you are going to be some kind of mere dev because trial and error are what you are going to be on all times.
Good try but most of the Algorithms are wayyyyyy too slow to use, for example, your sort algorithm is taking O(N^2) time too slow.
As ECMAScript doesn’t specify any sorting algorithm to be implemented by
Array.prototype.sort(), it totally depends on browser which sorting algorithm to be implement.Safari, Webkit, etc. uses 'Selection Sort' whereas Mozilla uses 'Merge Sort'.
The sorting algorithm behind
.fakeSort()mentioned in this article is 'Bubble Sort', and is intended for educational purposes and isn’t an efficient method for sorting in real world.Bubble Sort also is one of the most widely discussed algorithms, simply because of its lack of efficiency for sorting arrays. If an array is already sorted, Bubble Sort will only pass through the array once, however the worst case scenario is a run time of O(N²), which is extremely inefficient.
Are these implementations faster than native ones?
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