JavaScript
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Why this exists The first JavaScript Basics page covers variables and functions. The bugs that cost real time come from four rules underneath: where a variable is visible (scope), when it exists (hoisting), what a…
JavaScript Basics: Scope, Hoisting, Closures, this and Arrow Functions
The rules behind most "why is this undefined?" and "why does my loop print 3 three times?" bugs: var versus let and const, hoisting and the temporal dead zone, closures as private state, and how this is decided, with real output for every trap.
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