setInterval function can't read array

Question

Possible Duplicate:
Pass correct “this” context to setTimeout callback?

I have a problem with my little script.

function test() {
    this.arr = new Array('a','b','c');
    this.func = function() {
        //Do something with this.arr. But it's undefined.. WHY???
    }
    this.Start = function() {
        this.Interval = setInterval(this.func, 1000);
    }
}

var Test = new test();
Test.Start();

When I try to do anything with array in "func", it keeps telling me, that the array is undefined. Why?

why are you creating a bunch of variables? just do everything where it is needed and see if the variables are getting in the way due to `this` — Noah Passalacqua, Aug 05 '12 at 19:48
You capitalisation is odd. Usually only constructors are capitalised, and all other variables (including methods) are lowercase. — Bergi, Aug 05 '12 at 19:54

score 1 · Accepted Answer · edited Aug 05 '12 at 19:52

1

You're getting the wrong this reference, try:

function Test() {
  this.arr = new Array('a','b','c');
  this.func = function() {
    console.log(this.arr);
  };
  this.start = function() {
    var fun = this.func.bind(this);
    this.interval = setInterval(fun, 1000);
  };
}

var test = new Test();
test.start();

This article about this is a really interesting if you need more information: Function invocation and this

Also, note that I've changed the case on some symbols. Remember, functions that are meant to be used as constructors start with a uppercase letter, variables and methods use lowercase.

edited Aug 05 '12 at 19:52

Bergi

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answered Aug 05 '12 at 19:50

Pablo Fernandez

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While I agree with the style tips, I strongly suggest you give a reference, since there is no standards body (or BDFL) for JavaScript style. – kojiro Aug 05 '12 at 19:53

SReject · Answer 2 · 2012-08-06T07:19:41.090

1

Timers/Intervals are called on global scope so 'this' is set to 'window'. To get around this, you can pass a third option to setInterval() which will be used as a parameter for the specified function call.

function Test(){
    this.array = new Array('a','b','c');
    this.function = funct(){ }
    this.start = function(){
        this.Interval = setInterval(function(passedObj){
            passedObj.funct.call(passedObj);
        },1000,this);
    }
}
var testObj = new Test();
testObj.start();

Reading the comments below, you will see this is meant for nodejs. This, however, will not work for IE8 and/or earlier.

edited Aug 06 '12 at 07:19

answered Aug 05 '12 at 19:53

SReject

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What is `a`? This seems very wrong. – Bergi Aug 05 '12 at 19:55
1

The additional arguments to `setTimeout` aren't supported in IE8 and lower. – Aug 05 '12 at 19:55
I couldn't think of a descriptive name for it. unless I name it passedParam, which I felt was over doing it – SReject Aug 05 '12 at 19:56
...also using `function` as a property name will give you trouble in some implementations. – Aug 05 '12 at 19:57
Uh, haven't seen the additional argument. But a simple derefencing variable would be much easier. – Bergi Aug 05 '12 at 19:58
This is not a javascript for browser, but node.js – techbech Aug 05 '12 at 20:13
I meant to put 'funct', not function. cleaned up code, made mention that it isn't supported in IE8 or earlier. – SReject Aug 06 '12 at 07:21

setInterval function can't read array

2 Answers2