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. I will be continuously updating this list in a Github repository called . Please star and share if you want to follow along! JS Tips & Tidbits ⭐ 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 assigns variables by value. But this part is 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 is assigned. always very reference to the object in memory 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. var1 = ; var2 = var1;var2 = ; .log(var1); .log(var2); let 'My string' let 'My new string' console // 'My string' console // 'My new string' Let’s compare this with object assignment. var1 = { : } var2 = var1;var2.name = ; .log(var1); .log(var2); let name 'Jim' let 'John' console // { name: 'John' } console // { 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! { { .log(greeting + + name);
  }
} sayHello = createGreeter( );
sayHello( ); ( ) function createGreeter greeting return ( ) function name console ', ' const 'Hello' '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. { { fetch( );
  } { fetch(route, { : , : .stringify(params), : { : }
      })
  } { get, post }
} api = apiConnect( ); api.get( );
api.post( , { : }); ( ) function apiConnect apiKey ( ) function get route return ` ?key= ` ${route} ${apiKey} ( ) function post route, params return method 'POST' body JSON headers 'Authorization' `Bearer ` ${apiKey} return const 'my-secret-key' // No need to include the apiKey anymore 'http://www.example.com/get-endpoint' '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. obj = { : , : } { name, food } = obj; .log(name, food); const name 'Joe' food 'cake' const console // 'Joe' 'cake' If you want to extract properties under a different name, you can specify them using the following format. obj = { : , : } { : myName, : myFood } = obj; .log(myName, myFood); const name 'Joe' food 'cake' const name food console // '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! person = { : , : } { .log( );
} .log(introduce(person)); const name 'Eddie' age 24 ( ) function introduce { name, age } console `I'm and I'm years old!` ${name} ${age} console // "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. arr = [ , , , , , ]; max = .max(...arr); .log(max); const 4 6 -1 3 10 4 const Math console // 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! { .log(args[ ] + args[ ]);
}myFunc( , , , ); ( ) function myFunc ...args console 0 1 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 (my favorite JavaScript reference). all of them discussed on MDN 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. 1. map, filter, reduce return array where each element is transformed as specified by the function map: arr = [ , , , , , ]; mapped = arr.map( el + ); .log(mapped); const 1 2 3 4 5 6 const => el 20 console // [21, 22, 23, 24, 25, 26] return array of elements where the function returns true filter: arr = [ , , , , , ]; filtered = arr.filter( el === || el === ); .log(filtered); const 1 2 3 4 5 6 const => el 2 4 console // [2, 4] accumulate values as specified in function reduce: arr = [ , , , , , ]; reduced = arr.reduce( total + current); .log(reduced); const 1 2 3 4 5 6 const ( ) => total, current console // 21 The array methods find, findIndex, and indexOf can often be conflated. Use them as follows. 2. find, findIndex, indexOf : return the first instance that matches the specified criteria. Does not progress to find any other matching instances. find arr = [ , , , , , , , , , ]; found = arr.find( el > ); .log(found); const 1 2 3 4 5 6 7 8 9 10 const => el 5 console // 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! : This works almost identically to find, but rather than returning the first matching element it returns the of the first matching element. Take the following example, which uses names instead of numbers for clarity. findIndex index arr = [ , , , ]; foundIndex = arr.findIndex( el === ); .log(foundIndex); const 'Nick' 'Frank' 'Joe' 'Frank' const => el 'Frank' console // 1 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. indexOf: arr = [ , , , ]; foundIndex = arr.indexOf( ); .log(foundIndex); const 'Nick' 'Frank' 'Joe' 'Frank' const 'Frank' console // 1 There are a lot of great array method to help add or remove elements from arrays in a targeted fashion. 3. push, pop, shift, unshift 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. push: arr = [ , , , ]; pushed = arr.push( ); .log(arr); .log(pushed); let 1 2 3 4 const 5 console // [1, 2, 3, 4, 5] console // 5 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. pop: arr = [ , , , ]; popped = arr.pop(); .log(arr); .log(popped); let 1 2 3 4 const console // [1, 2, 3] console // 4 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. shift: arr = [ , , , ]; shifted = arr.shift(); .log(arr); .log(shifted); let 1 2 3 4 const console // [2, 3, 4] console // 1 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. unshift: arr = [ , , , ]; unshifted = arr.unshift( , , ); .log(arr); .log(unshifted); let 1 2 3 4 const 5 6 7 console // [5, 6, 7, 1, 2, 3, 4] console // 7 These methods either modify or return subsets of arrays. 4. splice, slice Change the contents of an array by removing or replacing existing elements and/or adding new elements. This method modifies the array in place. splice: The following code sample can be read as: at position 1 of the array, remove 0 elements and insert b. arr = [ , , , ];
arr.splice( , , ) let 'a' 'c' 'd' 'e' 1 0 'b' 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 modify the array in place but rather returns the desired subset. slice: does not arr = [ , , , , ]; sliced = arr.slice( , ); .log(sliced); .log(arr); let 'a' 'b' 'c' 'd' 'e' const 2 4 console // ['c', 'd'] console // ['a', 'b', 'c', 'd', 'e'] 5. 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. sort: arr = [ , , , , , , ]; sorter = firstEl - secondEl;
arr.sort(sorter); .log(arr); let 1 7 3 -1 5 7 2 const ( ) => firstEl, secondEl console // [-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. { ; ; ;
} greet = greeter(); .log(greet.next().value); .log(greet.next().value); .log(greet.next().value); .log(greet.next().value); * ( ) function greeter yield 'Hi' yield 'How are you?' yield 'Bye' const console // 'Hi' console // 'How are you?' console // 'Bye' console // undefined And using a generator for infinite values: { i = ; ( ) i++;
} ids = idCreator(); .log(ids.next().value); .log(ids.next().value); .log(ids.next().value); * ( ) function idCreator let 0 while true yield const console // 0 console // 1 console // 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. .log( == ); .log( === ); console 0 '0' // true console 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. joe1 = { : }; joe2 = { : }; .log(joe1 === joe2); const name 'Joe' const name 'Joe' console // 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). joe1 = { : }; joe2 = joe1; .log(joe1 === joe2); const name 'Joe' const console // 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! {
  setTimeout( {
    callback(text);
  }, );
}myFunc( , .log); ( ) function myFunc text, callback ( ) function 2000 'Hello world!' console // '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. myPromise = ( {
  setTimeout( { ( .random() < ) { res( );
    } rej( );
  }, );
});myPromise
  .then( { .log( + data);
   })
   .catch( { .log( + err);
   }); const new Promise ( ) function res, rej ( ) function if Math 0.9 return 'Hooray!' return 'Oh no!' 1000 ( ) function data console 'Success: ' ( ) function err console 'Error: ' // 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. greeter = ( {
  setTimeout( res( ), );
}) { greeting = greeter; .log(greeting);
}myFunc(); const new Promise ( ) => res, rej => () 'Hello world!' 2000 async ( ) function myFunc const await console // '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.

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