JavaScript is a complex language. If you’re a JavaScript developer at any level, it’s important to understand its foundational concepts. This article tackles 12 concepts that are critical for any JS developer to understand, but in no way represents the full breadth of what a JS developer should know.
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I will be continuously updating this list in a Github repository called JS Tips & Tidbits. Please star ⭐ and share if you want to follow along!
1. Value vs. Reference Variable Assignment
Understanding how JavaScript assigns to variables is foundational to writing bug-free JavaScript. If you don’t understand this, you could easily write code that unintentionally changes values.
JavaScript always assigns variables by value. But this part is very important: when the assigned value is one of JavaScript’s five primitive type (i.e., Boolean, null, undefined, String, and Number) the actual value is assigned. However, when the assigned value is an Array, Function, or Object a reference to the object in memory is assigned.
Example time! In the following snippet, var2 is set as equal to var1. Since var1 is a primitive type (String), var2 is set as equal to var1's String value and can be thought of as completely distinct from var1 at this point. Accordingly, reassigning var2 has not effect on var1.
let var1 = 'My string';let var2 = var1;
var2 = 'My new string';
console.log(var1);// 'My string'console.log(var2);// 'My new string'
Let’s compare this with object assignment.
let var1 = { name: 'Jim' }let var2 = var1;
var2.name = 'John';
console.log(var1);// { name: 'John' }console.log(var2);// { name: 'John' }
One might see how this could cause problems if you expected behavior like primitive assignment! This can get especially ugly if you create a function that unintentionally mutates an object.
2. Closures
Closure is an important JavaScript pattern to give private access to a variable. In this example, createGreeter returns an anonymous function that has access to the supplied greeting, “Hello.” For all future uses, sayHello will have access to this greeting!
function createGreeter(greeting) {return function(name) {console.log(greeting + ', ' + name);}}
const sayHello = createGreeter('Hello');sayHello('Joe');// Hello, Joe
In a more real-world scenario, you could envision an initial function apiConnect(apiKey) that returns some methods that would use the API key. In this case, the apiKey would just need to be provided once and never again.
function apiConnect(apiKey) {function get(route) {return fetch(`${route}?key=${apiKey}`);}
function post(route, params) {return fetch(route, { method: 'POST', body: JSON.stringify(params), headers: { 'Authorization': `Bearer ${apiKey}` } })}
return { get, post }}
const api = apiConnect('my-secret-key');
// No need to include the apiKey anymoreapi.get('http://www.example.com/get-endpoint');api.post('http://www.example.com/post-endpoint', { name: 'Joe' });
3. Destructuring
Don’t be thrown off by JavaScript parameter destructuring! It’s a common way to cleanly extract properties from objects.
const obj = {name: 'Joe',food: 'cake'}
const { name, food } = obj;
console.log(name, food);// 'Joe' 'cake'
If you want to extract properties under a different name, you can specify them using the following format.
const obj = {name: 'Joe',food: 'cake'}
const { name: myName, food: myFood } = obj;
console.log(myName, myFood);// 'Joe' 'cake'
In the following example, destructuring is used to cleanly pass the person object to the introduce function. In other words, destructuring can be (and often is) used directly for extracting parameters passed to a function. If you’re familiar with React, you probably have seen this before!
const person = {name: 'Eddie',age: 24}
function introduce({ name, age }) {console.log(`I'm ${name} and I'm ${age} years old!`);}
console.log(introduce(person));// "I'm Eddie and I'm 24 years old!"
4. Spread Syntax
A JavaScript concept that can throw people off but is relatively simple is the spread operator! In the following case, Math.max can't be applied to the arr array because it doesn't take an array as an argument, it takes the individual elements as arguments. The spread operator ... is used to pull the individual elements out the array.
const arr = [4, 6, -1, 3, 10, 4];const max = Math.max(...arr);console.log(max);// 10
5. Rest Syntax
Let’s talk about JavaScript rest syntax. You can use it to put any number of arguments passed to a function into an array!
function myFunc(...args) {console.log(args[0] + args[1]);}
myFunc(1, 2, 3, 4);// 3
6. Array Methods
JavaScript array methods can often provide you incredible, elegant ways to perform the data transformation you need. As a contributor to StackOverflow, I frequently see questions regarding how to manipulate an array of objects in one way or another. This tends to be the perfect use case for array methods.
I will cover a number of different array methods here, organized by similar methods that sometimes get conflated. This list is in no way comprehensive: I encourage you to review and practice all of them discussed on MDN (my favorite JavaScript reference).
**map, filter, reduce**There is some confusion around the JavaScript array methods map, filter, reduce. These are helpful methods for transforming an array or returning an aggregate value.
- map: return array where each element is transformed as specified by the function
const arr = [1, 2, 3, 4, 5, 6];const mapped = arr.map(el => el + 20);
console.log(mapped);// [21, 22, 23, 24, 25, 26]
- filter: return array of elements where the function returns true
const arr = [1, 2, 3, 4, 5, 6];const filtered = arr.filter(el => el === 2 || el === 4);
console.log(filtered);// [2, 4]
- reduce: accumulate values as specified in function
const arr = [1, 2, 3, 4, 5, 6];const reduced = arr.reduce((total, current) => total + current);
console.log(reduced);// 21
**find, findIndex, indexOf**The array methods find, findIndex, and indexOf can often be conflated. Use them as follows.
- find: return the first instance that matches the specified criteria. Does not progress to find any other matching instances.
const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];const found = arr.find(el => el > 5);
console.log(found);// 6
Again, note that while everything after 5 meets the criteria, only the first matching element is returned. This is actually super helpful in situations where you would normally break a for loop when you find a match!
- findIndex: This works almost identically to
find, but rather than returning the first matching element it returns the index of the first matching element. Take the following example, which uses names instead of numbers for clarity.
const arr = ['Nick', 'Frank', 'Joe', 'Frank'];const foundIndex = arr.findIndex(el => el === 'Frank');
console.log(foundIndex);// 1
- indexOf: Works almost identically to
findIndex, but instead of taking a function as an argument it takes a simple value. You can use this when you have simpler logic and don’t need to use a function to check whether there is a match.
const arr = ['Nick', 'Frank', 'Joe', 'Frank'];const foundIndex = arr.indexOf('Frank');
console.log(foundIndex);// 1
**push, pop, shift, unshift**There are a lot of great array method to help add or remove elements from arrays in a targeted fashion.
- push: This is a relatively simple method that adds an item to the end of an array. It modifies the array in-place and the function itself returns the item added to the array.
let arr = [1, 2, 3, 4];const pushed = arr.push(5);
console.log(arr);// [1, 2, 3, 4, 5]console.log(pushed);// 5
- pop: This removes the last item from an array. Again, it modifies the array in place. The function itself returns the item removed from the array.
let arr = [1, 2, 3, 4];const popped = arr.pop();
console.log(arr);// [1, 2, 3]console.log(popped);// 4
- shift: This removes the first item from an array. Again, it modifies the array in place. The function itself returns the item removed from the array.
let arr = [1, 2, 3, 4];const shifted = arr.shift();
console.log(arr);// [2, 3, 4]console.log(shifted);// 1
- unshift: This adds one or more elements to the beginning of an array. Again, it modifies the array in place. Unlike a lot of the other methods, the function itself returns the new length of the array.
let arr = [1, 2, 3, 4];const unshifted = arr.unshift(5, 6, 7);
console.log(arr);// [5, 6, 7, 1, 2, 3, 4]console.log(unshifted);// 7
**splice, slice**These methods either modify or return subsets of arrays.
- splice: Change the contents of an array by removing or replacing existing elements and/or adding new elements. This method modifies the array in place.
The following code sample can be read as: at position 1 of the array, remove 0 elements and insert b.
let arr = ['a', 'c', 'd', 'e'];arr.splice(1, 0, 'b')
- slice: returns a shallow copy of an array from a specified start position and before a specified end position. If no end position is specified, the rest of the array is returned. Importantly, this method does not modify the array in place but rather returns the desired subset.
let arr = ['a', 'b', 'c', 'd', 'e'];const sliced = arr.slice(2, 4);
console.log(sliced);// ['c', 'd']console.log(arr);// ['a', 'b', 'c', 'd', 'e']
sort
- sort: sorts an array based on the provided function which takes a first element and second element argument. Modifies the array in place. If the function returns negative or 0, the order remains unchanged. If positive, the element order is switched.
let arr = [1, 7, 3, -1, 5, 7, 2];const sorter = (firstEl, secondEl) => firstEl - secondEl;arr.sort(sorter);
console.log(arr);// [-1, 1, 2, 3, 5, 7, 7]
Phew, did you catch all of that? Neither did I. In fact, I had to reference the MDN docs a lot while writing this — and that’s okay! Just knowing what kind of methods are out there with get you 95% of the way there.
7. Generators
Don’t fear the *. The generator function specifies what value is yielded next time next() is called. Can either have a finite number of yields, after which next() returns an undefined value, or an infinite number of values using a loop.
function* greeter() {yield 'Hi';yield 'How are you?';yield 'Bye';}
const greet = greeter();
console.log(greet.next().value);// 'Hi'console.log(greet.next().value);// 'How are you?'console.log(greet.next().value);// 'Bye'console.log(greet.next().value);// undefined
And using a generator for infinite values:
function* idCreator() {let i = 0;while (true)yield i++;}
const ids = idCreator();
console.log(ids.next().value);// 0console.log(ids.next().value);// 1console.log(ids.next().value);// 2// etc...
8. Identity Operator (===) vs. Equality Operator (==)
Be sure to know the difference between the identify operator (===) and equality operator (==) in JavaScript! The ==operator will do type conversion prior to comparing values whereas the === operator will not do any type conversion before comparing.
console.log(0 == '0');// trueconsole.log(0 === '0');// false
9. Object Comparison
A mistake I see JavaScript newcomers make is directly comparing objects. Variables are pointing to references to the objects in memory, not the objects themselves! One method to actually compare them is converting the objects to JSON strings. This has a drawback though: object property order is not guaranteed! A safer way to compare objects is to pull in a library that specializes in deep object comparison (e.g., lodash’s isEqual).
The following objects appear equal but they are in fact pointing to different references.
const joe1 = { name: 'Joe' };const joe2 = { name: 'Joe' };
console.log(joe1 === joe2);// false
Conversely, the following evaluates as true because one object is set equal to the other object and are therefore pointing to the same reference (there is only one object in memory).
const joe1 = { name: 'Joe' };const joe2 = joe1;
console.log(joe1 === joe2);// true
Make sure to review the Value vs. Reference section above to fully understand the ramifications of setting a variable equal to another variable that’s pointing to a reference to an object in memory!
10. Callback Functions
Far too many people are intimidated by JavaScript callback functions! They are simple, take this example. The console.log function is being passed as a callback to myFunc. It gets executed when setTimeout completes. That’s all there is to it!
function myFunc(text, callback) {setTimeout(function() {callback(text);}, 2000);}
myFunc('Hello world!', console.log);// 'Hello world!'
11. Promises
Once you understand JavaScript callbacks you’ll soon find yourself in nested “callback hell.” This is where Promises help! Wrap your async logic in a Promise and resolve on success or reject on fail. Use “then” to handle success and catch to handle failure.
const myPromise = new Promise(function(res, rej) {setTimeout(function(){if (Math.random() < 0.9) {return res('Hooray!');}return rej('Oh no!');}, 1000);});
myPromise.then(function(data) {console.log('Success: ' + data);}).catch(function(err) {console.log('Error: ' + err);});
// If Math.random() returns less than 0.9 the following is logged:// "Success: Hooray!"// If Math.random() returns 0.9 or greater the following is logged:// "Error: On no!"
12. Async Await
Once you get the hang of JavaScript promises, you might like async await, which is just “syntactic sugar” on top of promises. In the following example we create an async function and within that we await the greeter promise.
const greeter = new Promise((res, rej) => {setTimeout(() => res('Hello world!'), 2000);})
async function myFunc() {const greeting = await greeter;console.log(greeting);}
myFunc();// 'Hello world!'
Conclusion
If you didn’t know any of these 12 concepts, you likely have grown at least a little in your knowledge of JavaScript! And if you knew them all, then hopefully this was a chance to practice and grow your knowledge. What other concepts do you think are critical? Let me know in the comments.