beginner
11 min read
Variables and Data Types in JavaScript
Every program you will ever write starts with the same move: storing information so you can use it later. A user's name, a score, a shopping cart total, a…
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JavaScript Fundamentals
Get comfortable with the building blocks behind JavaScript programs: values, variables, collections, comparisons, decisions, and repetition.
Tutorials
Articles in JavaScript Fundamentals
Follow these lessons in order, or jump directly to the topic you need.
beginner
9 min read
Basic Operators in JavaScript
You've learned how to store values in variables. Now it's time to make those values do something. JavaScript operators are the verbs of your code—they add,…
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beginner
9 min read
Working with Strings in JavaScript
A website is mostly text doing work: a signup form checking an email address, a search bar cleaning up what a user typed, a dashboard greeting someone by…
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beginner
9 min read
Arrays and Objects Basics
A variable can hold one value—and that value can be a collection holding many related values. JavaScript arrays and objects are how you build those…
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beginner
8 min read
Making Decisions with if/else
Every program you write has to react to the world around it. A login form needs to say "Welcome back" to one person and "Invalid password" to another. A…
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beginner
11 min read
Loops: for and while
Five console.log statements will print five numbers. Fifty will print fifty. Five hundred will print five hundred—if you have the patience to write them…
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beginner
12 min read
JavaScript Array Methods: map, filter, and reduce
You know how to build an array. Now comes the part where every beginner hits the same wall: you need to change the values, keep only some of them, or total…
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beginner
9 min read
let, const, and var in JavaScript: Which Should You Use?
You already know how to declare a variable. You've probably written let score = 0 or const userName = "Alex" and moved on. But then you open an older…
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beginner
7 min read
Fix "Assignment to Constant Variable" in JavaScript
Now you're stuck on a question that feels bigger than the error itself: do you change the const to let, or do you rewrite the line that's failing? The…
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beginner
9 min read
for Loops vs forEach vs map in JavaScript
You can loop over an array three different ways, and they all look nearly identical when you write them. So which one should you use?
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beginner
12 min read
JavaScript if/else and Boolean Practice Exercises
You can write an if/else statement. You know where the braces go and what console.log does. But when the condition gets even slightly tricky, you hesitate.…
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beginner
10 min read
JavaScript String Methods Reference for Beginners
You have text in a variable. You want to check it, cut it, clean it, or change it. But which method does which—and what does each one give you back?
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beginner
11 min read
Model Application Data with JavaScript Objects
You have probably already written code like this: a cartTotal here, a userName there, an isLoggedIn flag somewhere else. It works, right up until you…
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beginner
10 min read
Strict vs. Loose Equality in JavaScript: When to Use === and ==
Two equals signs and three equals signs look like a typo. They are not. They are two different questions, and one of them lies to you politely.
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beginner
7 min read
JavaScript Object Methods and this: Use Data Inside a Method
You copy a method into a new variable, call it, and get undefined instead of the name you expected. Nothing in the object changed. Nothing in the function…
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beginner
10 min read
JavaScript Set and Map: When to Use Each Collection
You have a list of tags with duplicates, or you want to count visits per user object, and your array or plain object starts fighting you. Here are the two…
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beginner
8 min read
JavaScript Scope: Global, Function, and Block Scope
You write a variable inside a function, log it, and it prints perfectly. You move that same console.log two lines down, outside the function, and suddenly…
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beginner
6 min read
JavaScript Closures: How Functions Remember Variables
A function can outlive the variables it was born next to. That sentence sounds wrong until you run it.
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beginner
9 min read
Recursion in JavaScript: Base Cases and Step-by-Step Tracing
That is the normal first experience with recursion. The function calls itself, the answer appears, and somewhere in the middle your mental model goes dark.…
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beginner
11 min read
JavaScript Destructuring and Spread Syntax for Arrays and Objects
Three lines, three property reads, three variables. It works, and it is honest code. But then you see a modern codebase do the same job in one line, with…
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beginner
9 min read
Handle JavaScript Errors with try, catch, and throw
One bad line can take down everything after it. Here is how to catch that failure at the exact point it happens, and how to throw your own when a function…
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beginner
8 min read
JavaScript Truthiness: Falsy Values and Conditional Checks
A condition does not ask whether a value is true. It asks what that value becomes when JavaScript forces it into a yes-or-no decision.
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beginner
10 min read
Work with JSON in JavaScript: Parse and Stringify Data
You log a value from an API or from localStorage, and it looks exactly like an object. You can see the keys. You can see the values. So you try to read a…
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beginner
7 min read
null vs. undefined in JavaScript: Meaning and Common Sources
You log three things in the console. A variable you declared but never filled in. A property you hoped was on an object. A function call that was supposed…
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beginner
9 min read
JavaScript Dates and Timestamps: Create, Inspect, and Understand Time
You type new Date() into the console and get back a long string with a time zone name you never asked for. You type Date.now() and get a big number. Same…
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beginner
10 min read
JavaScript Regular Expressions: Match and Check Simple Text
You start with a simple check. Does this input contain a dash? You reach for includes(). Then the input changes shape, and suddenly you have five if…
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