JavaScript enables interactive web pages and thus is an essential part of web applications. It is a language which is also characterized as dynamic, weakly typed, prototype-based and multi-paradigm.Īlongside HTML and CSS, JavaScript is one of the three core technologies of the World Wide Web. JavaScript (/ˈdʒɑːvəˌskrɪpt/), often abbreviated as JS, is a high-level, interpreted programming language.
#You dont know js scope closure how to#
Learn about scope, a set of rules to help JavaScript engines locate variables in your code Go deeper into nested scope, a series of containers for variables and functions Explore function- and block-based scope,?hoisting?, and the patterns and benefits of scope-based hiding Discover how to use closures for synchronous and asynchronous tasks, including the creation of JavaScript libraries. Armed with this knowledge, you can achieve true JavaScript mastery. Like other books in the “You Don’t Know JS” series, Scope and Closures dives into trickier parts of the language that many JavaScript programmers simply avoid. You’ll learn how and why they work, and how an understanding of closures can be a powerful part of your development skillset. This concise yet in-depth guide takes you inside scope and closures, two core concepts you need to know to become a more efficient and effective JavaScript programmer. Let DOES NOT! so move your lets to the top or get ready for errors.Ĭonst is block scoped as well, but it is a constant.You Don’t Know JS: Scope & Closures: No matter how much experience you have with JavaScript, odds are you don’t fully understand the language. VAR declarations hoist to the top of the scope when compiling Using an iife is a way to hide scope within functions pre es6. Use let in order to make sure that is true. Vars defined in a block (like a for statement, or an if statement) SHOULD be inaccessible outside that block Vars defined in a function are not accessible outside of that errorFunction IIFEs can end in )() OR ()) They both work the same way. and it is hard or impossible to make it recursively call itself. anonymous is harder to read in stack traces, harder to add / remove handlers or timeouts on this is the () part, and it may be optional.įunction expression – the name does not exist in the global namespace, Only in the parens. IIFE! immediately invoked function expression. What is the module pattern? how is it defensive and prevent namespace collisions? The program will not ‘see’ the global scope so it will overwrite it while in the inner Scope. Scope shadowing is when you redefine (using var let or const) a global scope var inside a inner level Scope. TypeError: you’re doing something dumb with the variable. ReferenceError : the var is not found in this Scope That Scope resolution was successful, but that there was an illegal/impossible “ReferenceError is Scope resolution-failure related, whereas TypeError implies I’m not sure if I don’t understand it, or if I just misunderstand why people think it is hard to understand. I learned a bunch of things about hoisting, and a bit more about closures. After 2 hours my eyes glazed over, but it was really great while it lasted. 2h Tonight I got into reading you don’t know js on scope and closures.