javascript - Whats the proper way to iterate through an object? -

i've used

for( x in object ){} 

but came accros

for( var x in object){} 

i know var matters in regular

for( var i=0;i<10;i++){} 

statements, bit me once in recursive function, matter when using x in?...

re var part:

does matter when using x in?

yes. always matters declare variables. if don't, , you're using loose mode, fall prey the horror of implicit globals. (in strict mode, it's nice detectable error. use strict mode.) bite in recursive function, , in of large number of other non-recursive situations.

you don't have declare within for construct, make sure declare somewhere.

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re "whats proper way iterate through object?":

there several proper ways. like mathletics, i'm partial using object.keys(...).foreach(...):

var obj = {    a: "ay",    b: "bee",    c: "see"  };  object.keys(obj).foreach(function(key) {    snippet.log(key + ": " + obj[key]);  });

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note object.keys @ object's own properties, not properties of prototype.

the reason gives me nice self-contained scope i'm doing on each "loop" iteration. if you're worried runtime cost of callbacks, don't be.

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but can use for in loop. if do, may or may not want include hasownproperty, depending on whether want visit prototype properties.

var proto = {    a: "ay (proto)",    b: "bee (proto)",    c: "see (proto)",    d: "dee (proto)"  };  var obj = object.create(proto);  obj.a = "ay";  obj.b = "bee";  obj.c = "see";    // includes prototype properties if not shadowed object's own  snippet.log("for-in without hasownproperty");  var key;  (key in obj) {    snippet.log(key + ": " + obj[key]);  }    // not include prototype properties  snippet.log("for-in using obj.hasownproperty");  (key in obj) {    if (obj.hasownproperty(key)) {      snippet.log(key + ": " + obj[key]);    }  }    // if you're feeling paranoid `obj` having had  // `hasownproperty` overridden, can use `object.prototype`:  snippet.log("for-in using object.prototype.hasownproperty");  (key in obj) {    // not include prototype properties    if (object.prototype.hasownproperty.call(obj, key)) {      snippet.log(key + ": " + obj[key]);    }  }  // i've never needed in real world, though

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just don't use for-in on arrays without using appropriate safeguards; more why (and alternatives) in this answer.