JavaScript loop variable scope

See the MDN for the "initialization parameters" of a for-loop:

An expression (including assignment expressions) or variable declaration. Typically used to initialize a counter variable. This expression may optionally declare new variables with the var keyword. These variables are not local to the loop, i.e. they are in the same scope the for loop is in. The result of this expression is discarded.

The javascript folks are trying to fix this!

EcmaScript6 (aka EcmaScript 2015) is the latest version of javascript that was passed last summer and browsers are just starting to support its features.

One of those features is block-scope local variables with the "let" expression. As of right now (April 2016), most of the current versions of the major browsers support this except Safari. Few mobile browsers support this.

You can read more about it here (in particular, see the section "let-scoped variables in for loops"): https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let

You can check current browser support here (look for the row Bindings -> let): https://kangax.github.io/compat-table/es6/

JavaScript didn't have block scope until const and let were introduced, just var which is function scoped. Since the initialization of i is within one function, that variable is accessible anywhere else in that same function.

From MDN:

Important: JavaScript does not have block scope. Variables introduced with a block are scoped to the containing function or script, and the effects of setting them persist beyond the block itself. In other words, block statements do not introduce a scope. Although "standalone" blocks are valid syntax, you do not want to use standalone blocks in JavaScript, because they don't do what you think they do, if you think they do anything like such blocks in C or Java.