[removed] Iterating over array with non-consecutiv

Question

I need to iterate over an array for which the keys are non-consecutive:

var messages = new Array();
messages[0] = "This is the first message";
messages[3] = "This is another message";

Obviously using the index of a for loop will not work as it depends on the keys being sequential:

for (var i=0 ; i<messages.length ; i++) {
    alert(messages[i]); // Will only alert the first message, as i is never equal to 3
}

What is the canonical way of dealing with this, seeing as the for-each syntax is not intended for iterating over values in an array in javascript? Thanks.

Answer 1

For a use case such with the assumptions:

array.length >== 1 (i.e: an array with meaningful data in it already)

Where you are interested in the data from array[1], array[15], array[45] etc

You can do something similar to:

var array = ["you","will","become","strong","with","the","codes","padawan"];
var values = [1,5,7];


for (var i = 0; i < values.length; i++){
    var cypher = values[i];
    console.log(array[cypher]);
}
//will, the, padawan

Or perhaps something more meaningful such as:

for (var i = 0; i < values.length; i++){
    var cypher = values[i];
    aService.aFn.(array[cypher],cb);
}
//calls aService.aFn separately for each value array[1] , array[5] , array[7] passed as args

Answer 2

You can use each() jQuery method to do this.

$.each(messages, function(index, val){
    alert(val); 
});

From jQuery docs

each()

A generic iterator function, which can be used to seamlessly iterate over both objects and arrays. Arrays and array-like objects with a length property (such as a function's arguments object) are iterated by numeric index, from 0 to length-1. Other objects are iterated via their named properties.

Answer 3

When you create an array and give it values at 0 and 3, undefined values are created at 1 and 2. try this:

$.each(messages, function(i,val) { 
  if (val) {
    alert(val);
  } 
});

Answer 4

The idiomatic way would be to use an object, not an array. Just be sure to check hasOwnProperty to make sure you don't pick up stray things which may have been added to the prototype.

var messages = { };
messages[0] = "This is the first message";
messages[3] = "This is another message";

for (var i in messages) {
    if (messages.hasOwnProperty(i))
        alert(messages[i]); 
}

---

Or, the more modern way would be to use Object.keys

Object.keys(messages).forEach(prop => {
    alert(messages[prop]);
});

Be sure to transpile that code with Babel if you plan on running it in older browsers like IE.

Answer 5

for(var i in messages)
{
    console.log(messages[i]);
}

Answer 6

Simple! if the array has regular gaps between the indices do this:

for (var i = 0 ; i < messages.length; i += gap) {
    alert(messages[i]); // Will only alert the messages at the regular interval/gap 
}