How to get Javascript array length?

What is the best way to get output from a 2 layer loop for 2D array in Javascript?

• Here is my code: var myArray = {}; myArray["abc"] = new Array(); myArray["def"] = new Array(); myArray["abc"][0] = "John"; myArray["abc"][1] = "Mary"; myArray["abc"][2] = "Jack"; var letter = ""; var id = ""; for (var i_letter in myArray) { for (var j_id = 0; j_id < myArray[i_letter].length; j_id++) { if (myArray[i_letter][j_id]=== "Mary") { letter = i_letter; // error ? id = j_id; // error? } } } document.write( letter + “ , ” + id); // why error? Suppose to be “abc , 1” What is the best way to get output, namely i_letter and j_id, from outside the loop?  By the way, if myArray contains huge data, is there anyway to write codes more efficiently?

• Answer:

  First of all myArray as you defined it is not a JavaScript array but a JavaScript object and then do not provide JavaScript Array methods The Array like notation in is used in JS to set dynamically the name of the object property you want to get In this code myArray = {}; // object literal notationmyArray["abc"] = new Array(); // similar to myArray.abc = new Array(); This notation would have let you define the "abc" property name in a variable var indexName = "abc";myArray[indexName] = new Array(); And the property name could even be the result of an expression myArray[indexName + i] = new Array(); In JS {} represent an object and [] represent an array Note that it is a better practice to use the literal array notation than new Array() myArray.abc = [] // similar to myArray.abc = new Array(); A potentially better version of your code, keeping myArray as an object (I'll call it "data"), could be var data = { // was "myArray" abc: ["John", "Mary", "Jack"], // new Array() with items 0, 1, and 2 def: [] // new Array()};var result;var arrayNames = Object.keys(data); // get data property names in an arrayarrayNames.forEach(function (name) { // parse the propNames array items data[name].some(function (value, index) { // parse array value until item found if (value === "Mary") { result = [name, index]; return true; } return false; });});console.log(result); About the error in your code it comes from the fact that in this line: document.write( letter + “ , ” + id); The used double quote (") character is not the valid JS (ASCII) double quote character. This version would work better: document.write( letter + " , " + id); [EDIT] I realize I didn't talked about the context of huge array So, if you have a huge array such code may be blocking the UI as the Array forEach is nor asynchronous. This lib provides a way to do Async forEach on arrays: https://github.com/cowboy/javascript-sync-async-foreach. It would nor result in better "performances but at least would make the interface still responsible. You could even show an advancement status via a progress bar if needed. If you're arrays are so big, you may consider to index your values, reproducing btree or cluster indexes is not that hard in JS. Note that you might consider to use IndexedDB (with a polyfil for Safari) or also split your arrays in different shared workers trough which you could implement a map/reduce mechanism