add element to array with string key in jquery

Question

I had already functioning code. Some part of the code should change two values of queryParams array:

myParam1 = 'sort_by';
myParam2 = 'sort_order';
queryParams[myParam1] = 'title';
if (queryParams[myParam2] == 'ASC') {
    queryParams[myParam2] = 'DESC';
} else { //DESC or undefined
    queryParams[myParam2] = 'ASC';
}
queryPairs = [];
for (var index in queryParams) {
    queryPairs.push(index + "=" + queryParams[index]);
}

Unfourtunately some logic has changed and now sometimes queryParams has length 0 at the beginning of this script and then this part failes.

queryParams[myParam1] = 'title';

and

queryParams[myParam2] = 'DESC'

lines do not change queryParams.length,so the length is still 0 and the loop

for (var index in queryParams){

do not work as expected.

I have not found how to add key/value into the array.

T.J. Crowder · Accepted Answer · 2013-09-12 10:38:34Z

5

You're being a bit confused by the fact that JavaScript's standard arrays aren't really arrays at all.

You're not using queryParams as an array in the code you've quoted. You're using it as an object. Objects don't have a length property by default.

You haven't said what changed, so it's difficult to help you, but note that even if queryParams is an array and you add a property to it as you are, the length will remain 0. The length of an array only relates to a special class of properties (ones that meet the definition of an "array index"), not to all properties.

Some examples that may help:

var a = []; // a is an array
console.log(a.length); // "0"
a['foo'] = 'bar';
console.log(a.length); // "0", the `foo` property doesn't have any effect on `length`
a[0] = 'zero';
console.log(a.length); // "1", the `0` property *does* have an effect on `length`

var o = {}; // o is an object
var myParam1 = 'sort_by';
var myParam2 = 'sort_order';
var propertyName;

o[myParam1] = 'foo';
o[myParam2] = 'DESC';

for (propertyName in o) {
    console.log(propertyName); // "sort_by" and then "sort_order" (the order is not defined)
}

answered Sep 12, 2013 at 10:38

T.J. Crowder

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2 Comments

renathy Over a year ago

Before changes queryParams[myParam1] and queryParams[myParam2] always were defined, so the code just changed values of those two items. But after changes, there are situations when queryParams[myParam1] and queryParams[myParam2] are undefined and in this case just writing queryParams[myParam2] = 'DESC' failes, because it doesn't appear in for loop. I need it to appearin for loop, what should I do?

T.J. Crowder Over a year ago

@renathy: That assignment shouldn't fail, not if queryParams is an object (plain object or array, either way). It may be that queryParams isn't initialized at all, in which case you can do this: if (!queryParams) { queryParams = {}; } before assigning to its properties. Another way you'll see that written is queryParams = queryParams || {};

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