What kind of blocks do create closure in Javascript -

What kind of blocks do create closure in Javascript -

after working long javascript, i'm still having problems coming terms closure!

class="snippet-code-js lang-js prettyprint-override">(function() { var pages = ["one", "two", "three"]; (var index in pages) { var p = pages[index]; settimeout(function() { console.log(p); }, 500); } })();

shouldn't above snippet print one, two , three? afaik p within closure created for block , should remain unchanged each of settimeouts pick them up! why printing 3 threes?

javascript has function scope, not block scope. so, when looping on array (p.s. don't utilize for..in arrays), creating 3 closures. each closure create referencing same p variable. when timeouts run, all print "three" because printing same p variable.

you need create new scope each element in array. lucky, javascript has built-in function this, .foreach.

class="snippet-code-js lang-js prettyprint-override">(function() { var pages = ["one", "two", "three"]; pages.foreach(function(p){ settimeout(function() { console.log(p); }, 500); }); })();

when runs, runs callback each element. when callback ran, new scope created each element, works properly.

javascript closures