Download e-book for kindle: You Don't Know JS: Scope & Closures by Kyle Simpson

By Kyle Simpson

ISBN-10: 1449335535

ISBN-13: 9781449335533

Regardless of how a lot adventure you might have with JavaScript, odds are you don’t absolutely comprehend the language. This concise but in-depth consultant takes you within scope and closures, center options you want to recognize to turn into a extra effective and powerful JavaScript programmer. You’ll learn the way and why they paintings, and the way an figuring out of closures could be a robust a part of your improvement skillset.

Like different books within the "You Don’t be aware of JS" sequence, Scope and Closures dives into trickier elements of the language that many JavaScript programmers easily steer clear of. Armed with this data, you could in attaining real JavaScript mastery.

* know about scope, a suite of principles to assist JavaScript engines find variables on your code
* move deeper into nested scope, a sequence of bins for variables and functions
* discover functionality- and block-based scope, “hoisting”, and the styles and advantages of scope-based hiding
* realize find out how to use closures for synchronous and asynchronous initiatives, together with the production of JavaScript libraries

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Extra info for You Don't Know JS: Scope & Closures

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Cheat) lexical scope at runtime? JavaScript has two such mechanisms. Both of them are equally frowned upon in the wider community as bad practices to use in your code. But the typical arguments against them are often missing the most important point: cheating lexical scope leads to poorer perfor‐ mance. Before I explain the performance issue, though, let’s look at how these two mechanisms work. ) function in JavaScript takes a string as an argument and treats the contents of the string as if it had actually been authored code at that point in the program.

There’s a variety of reasons motivating this scope-based hiding. They tend to arise from the software design principle Principle of Least Privilege1, also sometimes called Least Authority or Least Exposure. This principle states that in the design of software, such as the API for a module/object, you should expose only what is minimally necessary, and “hide” everything else. This principle extends to the choice of which scope to contain variables and functions. If all variables and functions were in the global scope, they would of course be accessible to any nested scope.

Avoid! " ); } })(); Still another variation of the IIFE inverts the order of things, where the function to execute is given second, after the invocation and pa‐ rameters to pass to it. This pattern is used in the UMD (Universal Module Definition) project. Some people find it a little cleaner to un‐ derstand, though it is slightly more verbose. a ); // 2 }); The def function expression is defined in the second-half of the snip‐ pet, and then passed as a parameter (also called def) to the IIFE func‐ tion defined in the first half of the snippet.

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You Don't Know JS: Scope & Closures by Kyle Simpson


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