JavaScript In Practice: ES6 And Beyond/

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var vs let

var vs let

introduction to the `let` keyword for declaring block-scoped variables; and the dangers of scoping, such as the temporal dead zone

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Variables declared with var have function scope. This means that they are accessible inside the function/block they are defined in. Take a look at the following code:

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var guessMe = 2;
console.log("guessMe: "+guessMe);// A: guessMe is 2
( function() {
    console.log("guessMe: "+guessMe);// B: guessMe is undefined
    var guessMe = 5;
    console.log("guessMe: "+guessMe);// C: guessMe is 5
} )();
console.log("guessMe: "+guessMe);// D: guessMe is 2

Comment B may surprise you if you have not heard of hoisting. If a variable is declared using var inside a function, the Javascript engine treats them as if they are declared at the top ...