Looping through JSON with node js

Question

I have a JSON file which I need to iterate over  as shown below            device id    8020        data               Timestamp    04-29-11 05 22 39 pm            Start Value    0 02           Abstract   18 60           Editor   65 20                   Timestamp    04-29-11 04 22 39 pm            End Value    22 22           Text   8 65           Common   1 10           Editable    true            Insert   6 0            The keys in data will not always be the same  i ve just used examples  there are 20 different keys   and as such  I cannot set up my script to statically reference them to get the values   Otherwise I could state  var value1   json data Timestamp  var value2   json data Start Value  var value3   json data Abstract  etc   In the past i ve used a simple foreach loop on the data node     foreach   json- gt data as  key   gt   val        switch  key            case  Timestamp                 do this          case   Start Value                 do this           But don t want to block the script  Any ideas

User · Answer

Take a look at Traverse   It will recursively walk an object tree for you and at every node you have a number of different objects you can access - key of current node  value of current node  parent of current node  full key path of current node  etc   https   github com substack js-traverse   I ve used it to good effect on objects that I wanted to scrub circular references to and when I need to do a deep clone while transforming various data bits   Here s some code pulled form their samples to give you a flavor of what it can do   var id   54  var callbacks       var obj     moo   function        foo    2 3 4  function            var scrubbed   traverse obj  map function  x        if  typeof x      function             callbacks id      id   id  f   x  path   this path            this update   Function             id               console dir scrubbed   console dir callbacks

User · Answer

If we are using nodeJS  we should definitely take advantage of different libraries it provides  Inbuilt functions like each    map    reduce   and many more from underscoreJS reduces our efforts  Here s a sample       var   require  underscore        var fs require  fs         var jsonObject JSON parse fs readFileSync  YourJson json    utf8             map  jsonObject  function content              map content function data              if data Timestamp                console log data Timestamp

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My most preferred way is   var objectKeysArray   Object keys yourJsonObj  objectKeysArray forEach function objKey        var objValue   yourJsonObj objKey

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You may also want to use hasOwnProperty in the loop   for  var prop in obj        if  obj hasOwnProperty prop             switch  prop                   obj prop  has the value                     node js is single-threaded which means your script will block whether you want it or not  Remember that V8  Google s Javascript engine that node js uses  compiles Javascript into machine code which means that most basic operations are really fast and looping through an object with 100 keys would probably take a couple of nanoseconds    However  if you do a lot more inside the loop and you don t want it to block right now  you could do something like this  switch  prop        case  Timestamp           setTimeout function            5           break      case  Start Value           setTimeout function            10           break      If your loop is doing some very CPU intensive work  you will need to spawn a child process to do that work or use web workers

User · Answer

You can iterate through JavaScript objects this way   for var attributename in myobject       console log attributename      myobject attributename        myobject could be your json data

User · Answer

I would recommend taking advantage of the fact that nodeJS will always be ES5  Remember this isn t the browser folks you can depend on the language s implementation on being stable  That said I would recommend against ever using a for-in loop in nodeJS  unless you really want to do deep recursion up the prototype chain  For simple  traditional looping I would recommend making good use of Object keys method  in ES5  If you view the following JSPerf test  especially if you use Chrome  since it has the same engine as nodeJS   you will get a rough idea of how much more performant using this method is than using a for-in loop  roughly 10 times faster   Here s a sample of the code    var keys   Object keys  obj     for  var i   0 length   keys length  i  lt  length  i            obj  keys  i

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Not sure if it helps  but it looks like there might be a library for async iteration in node hosted here https   github com caolan async     Async is a utility module which provides straight-forward  powerful functions for working with asynchronous JavaScript  Although originally designed for use with node js  it can also be used directly in the browser       Async provides around 20 functions that include the usual  functional  suspects  map  reduce  filter  forEach     as well as some common patterns for asynchronous control flow  parallel  series  waterfall      All these functions assume you follow the node js convention of providing a single callback as the last argument of your async function

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A little late but I believe some further clarification is given below  You can iterate through a JSON array with a simple loop as well  like  for var i   0  i  lt  jsonArray length  i          console log jsonArray i  attributename      If you have a JSON object and you want to loop through all of its inner objects  then you first need to get all the keys in an array and loop through the keys to retrieve objects using the key names  like  var keys   Object keys jsonObject   for var i   0  i  lt  keys length  i           var key   keys i       console log jsonObject key attributename

User · Answer

If you want to avoid blocking  which is only necessary for very large loops  then wrap the contents of your loop in a function called like this  process nextTick function    lt contents of loop gt     which will defer execution until the next tick  giving an opportunity for pending calls from other asynchronous functions to be processed

[json] Looping through JSON with node.js

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