Push to a javascript array if it exists, if not then create it first

Question

Is there a way for this line to always work and not throw TypeError: Cannot read property 'Whatever' of undefined

var MyArray = [];
MyArray[StringVariableName][StringVariableName2].push("whatever");

`MyArray = MyArray || [];` – Cory Danielson Feb 18 '14 at 16:15 — Cory Danielson, Feb 18 '14 at 16:15

score 19 · Accepted Answer · answered Feb 18 '14 at 16:14

19

Try this:

var MyArray = [];
MyArray[StringVariableName] = MyArray[StringVariableName] || [];
MyArray[StringVariableName][StringVariableName2] = MyArray[StringVariableName][StringVariableName2] || [];
MyArray[StringVariableName][StringVariableName2].push("whatever");

answered Feb 18 '14 at 16:14

ppoliani

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Works amazing, can you explain what that assignment means? – Nadav Miller Feb 18 '14 at 16:30
@NadavMiller It simply is a logical OR. If first part is true, second part is not executed and first part is assigned to the variable. – Sachin Jain Feb 18 '14 at 16:31
It is the logical OR operator and it simply says: If the left operand is a truthy value then return it otherwise return the right operand. – ppoliani Feb 18 '14 at 16:33

score 3 · Answer 2 · answered Aug 15 '16 at 14:23

3

You could even, through the power of expressions, do this with a one-liner.

(MyArray[StringVariableName][StringVariableName2] || (MyArray[StringVariableName][StringVariableName2] = [])).push("whatever");

answered Aug 15 '16 at 14:23

Jack

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This is awesome. Is there any performance lag in this approach? – Sajith Mantharath Sep 09 '21 at 11:25

Sachin Jain · Answer 3 · 2014-02-18T16:48:55.297

0

I think instead of using array in the first place, use object if your keys are not integers. In Javascript Arrays are also object So it is not wrong to do this

var a = [];
a['key'] = 'something';

console.log(a); //Gives []

I think it is conceptually wrong So instead of using Array to hold such pair of data you should use objects. See this:

var myObject = myObject || {};
myObject[str1] = myObject[str1] || {};
myObject[str1][str2] = myObject[str][str2] || [];

// Now myObject[str1][str2] is an array. Do your original operation

myObject[str1][str2].push("whatever");

edited Feb 18 '14 at 16:48

answered Feb 18 '14 at 16:21

Sachin Jain

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.push() is for arrays, not objects. – adrien Oct 23 '17 at 18:26
@A__ Both of us are correct as `myObject[str1][str2]` is an array and not an object here. – Sachin Jain Oct 24 '17 at 05:21

score 0 · Answer 4 · answered Feb 18 '14 at 16:27

0

You could use the literal syntax to set things up like you'd have them:

var myObj = {
    StringVariableName: {
        StringVariableName2: []
    }
};

myObj.StringVariableName.StringVariableName2.push("whatever");

answered Feb 18 '14 at 16:27

Laoujin

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You are doing it wrong way because StringVariableName looks like actually a variable name. You are making them as constant keys in the object. – Sachin Jain Feb 18 '14 at 16:30

score 0 · Answer 5 · edited May 23 '17 at 12:09

To check without getting an error:

this snippet allows you to check if a chained object exists.

var x;
try{x=MyArray[name1][name2][name3][name4]}catch(e){}
!x||(x.push('whatever'));

from

https://stackoverflow.com/a/21353032/2450730

Shorthand creation of object chains in Javascript

this function allows you to create chained objects with a simple string.

function def(a,b,c,d){
 c=b.split('.');
 d=c.shift();//add *1 for arrays
 a[d]||(a[d]={});//[] for arrays
 !(c.length>0)||def(a[d],c.join('.'));
}

usage

var MyArray={};//[]
def(MyArray,'name1.name2.name3.name4');//name1+'.'+name2....

from

https://stackoverflow.com/a/21384869/2450730

both work also for arrays with a simple change.replace {} with []

if you have any questions just ask.

Push to a javascript array if it exists, if not then create it first

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