CODEWITHSUNDEEP
javascriptobjectproperties
thelolcat
thelolcat

Reputation: 11545

Add properties to object

I'm trying to set some object properties, but I get this error sometimes:

Uncaught TypeError: Cannot read property '4' of undefined

The code:

var obj = {};

obj['fields'][4]['name'] = value;

yes, 4 does not exist, but why it is not created?

Upvotes: 0

Views: 113

Answers (3)

Matt
Matt

Reputation: 41832

Actually, it's telling you obj['fields'] is undefined. And no, it does not automatically create nesting for you.

var obj = {};
obj['fields'] = {};
obj['fields'][4] = {};
obj['fields'][4]['name'] = value;

When working with an object, if you are unsure whether or not it has it's nesting already (and you don't want to overwrite it if it does), you can do the following:

obj['fields'] = obj['fields'] || {}
obj['fields'][4] = obj['fields'][4] || {};
//etc

To check for undefined:

if(!obj['fields']){} // undefined returns falsey
//or 
if('fields' in obj == false){} // notice you can't lead with ! here
//or
if(obj['fields'] === undefined){} // note the triple ===

Upvotes: 8

Guffa
Guffa

Reputation: 700152

Trying to read a non-existing property doesn't create it. The exact reason that you get an error is that trying to access fields returns undefined, and you can't access any properties from an undefined value.

To set the value, you need to create all parents:

obj.fields = [];
obj.fields[4] = {};
obj.fields[4].name = value;

Upvotes: 0

Carlos
Carlos

Reputation: 238

You have to init the the properties first:

var obj = {};

obj['fields'] = new Array();
obj['fields'][4] = new Array();
obj['fields'][4]['name'] = value;

Upvotes: 1

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