#You dont know js hoisting code#
#You dont know js hoisting series#

#You dont know js hoisting code#

Code will run slower as a result of using either feature. The downside to these mechanisms is that it defeats the Engine's ability to perform compile-time optimizations regarding scope look-up, because the Engine has to assume pessimistically that such optimizations will be invalid. The latter essentially creates a whole new lexical scope (again, at runtime) by treating an object reference as a 'scope' and that object's properties as scoped identifiers. The former can modify existing lexical scope (at runtime) by evaluating a string of 'code' which has one or more declarations in it. Two mechanisms in JavaScript can 'cheat' lexical scope: eval(.) and with. The lexing phase of compilation is essentially able to know where and how all identifiers are declared, and thus predict how they will be looked-up during execution. Lexical scope means that scope is defined by author-time decisions of where functions are declared. Unfulfilled LHS references result in an automatic, implicitly-created global of that name (if not in 'Strict Mode' ), or a ReferenceError (if in 'Strict Mode' ). Unfulfilled RHS references result in ReferenceErrors being thrown.

Later, a = 2 to look up the variable (LHS reference) and assign to it if found.Д«oth LHS and RHS reference look-ups start at the currently executing Scope, and if need be (that is, they don't find what they're looking for there), they work their way up the nested Scope, one scope (floor) at a time, looking for the identifier, until they get to the global (top floor) and stop, and either find it, or don't. This is performed at the beginning, before code execution. The JavaScript Engine first compiles code before it executes, and in so doing, it splits up statements like var a = 2 into two separate steps:ДЇirst, var a to declare it in that Scope. Scope-related assignments can occur either with the = operator or by passing arguments to (assign to) function parameters. LHS references result from assignment operations. This look-up may be for the purposes of assigning to the variable, which is an LHS (left-hand-side) reference, or it may be for the purposes of retrieving its value, which is an RHS (right-hand-side) reference. Scope is the set of rules that determines where and how a variable (identifier) can be looked-up. Scope & Closures Chapter 1: What is Scope?

Up & Going - intro level stuff (put at the end.just because).

To dig through everything (if you're a serious front-end or web developer you should probably read the whole series.unless you know everything all ready!)Д«IG THANKS to: Kyle Simpson the author of the You Don't Know JS 1st Edition (book series), you gave me a Promise.to save me from callback hell.

#You dont know js hoisting series#

This is a great way for higher level devs to still get the benefits of the series without having This can be used to quickly reviewД®ither certain sections or the entire book. This is meant to be a quick overview of everything contained with-in the YDKJS book series. A quick Overview of the freaking awesome You Don't Know JS 1st Edition (book series).