JavaScript: Why does my object property equal undefined?

Question

Hi I'm a 14 year old programmer. I just spent some time to create a mod for Minecraft Pocket Edition using something called ModPe. ModPe supplies me with a bunch of functions which I can use together, with JavaScript. Anyway I see nothing wrong in my code, this is why I come here. Here it is:

if (entityIsPassiveMob(entityId)) { // only add entity to list of entitys if entity is a passive mob
    var entityData = 1; // variable to be used with properties, it is set to 1 to become an object. An exception can't have a property because its not an object.
    entityData.flyType = random(1, 4); // 1 = rocketers, 2 = magical, 3 = dizzy, 4 = tired
    entityData.rocketers = [];
    entityData.magical = [random(1, 10)]; // amountBlocksAboveGround
    entityData.dizzy = [];
    entityData.tired = random(1, 4); // amountBlocksAboveGround
    listEntitys.push([entityId, entityData]); // push needed data into array
    clientMessage("added entity as " + entityData.flyType); // this prints undefined in Minecraft PE's chat box :/ 
}

I appreciate your help! The object property flyType is basically undefined, don't know what the others are equal to, but most likely undefined as well.

The Paramagnetic Croissant · Accepted Answer · 2014-11-16 06:31:09Z

2

entityData is not an object. It's a number. Since a number is a primitive, it can't have properties. Consequently, when you access (read/write) one of its properties, a temporary, auto-boxed Number object is created, and it's thrown away immediately (after the expression is evaluated).

Thus, you are not accessing the same object, rather different temporary objects, when operating on your entityData.

The solution: make it into a proper non-primitive object instead:

var entityData = {};

If you want to be elegant as well, you can initialize it with default properties, since object literals permit that too:

var entityData = {
    flyType: random(1, 4),
    rocketers: [],
    magical: [ random(1, 10) ]
};

edited Nov 16, 2014 at 6:31

answered Nov 15, 2014 at 23:10

The Paramagnetic Croissant

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5 Comments

RobG Over a year ago

Might want to check that syntax. ;-) flyType : random(1, 4),

The Paramagnetic Croissant Over a year ago

@RobG ugh, thanks for the correction! it was late in the night and I messed up the syntax. Fixed!

Jaskaran Singh Over a year ago

It's already upvoted, if your asking me. I clicked the tick next to your question so it's green and the second arrow above the tick. I've only asked two questions on this site, correct me if I'm wrong!

The Paramagnetic Croissant Over a year ago

@JaskaranSingh No, I am not asking you. I'm asking whomever downvoted…

RobG Over a year ago

@TheParamagneticCroissant—get used to it. There are drive-by down voters who are too lazy to bother saying why they voted as they did. An up–vote gives you 5x the reputation anyway. ;-)

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