I encounter an error in the following code because of the way I try to define the 'scanning' property on my reader object.
app.controller("turingController", ["$scope", function($scope) {
$scope.cellArray = [];
for( var i=0; i<=999; i++) {
$scope.cellArray[i] = {
index: i,
content: "B",
};
};
$scope.reader = {
location: 500,
state: "q1",
scanning: $scope.cellArray[$scope.reader.location].content
};
}]);
But if I don't use object literal notation to define the 'scanning' property, it works fine. Like this:
$scope.reader = {
location: 500,
state: "q1",
};
$scope.reader.scanning = $scope.cellArray[$scope.reader.location].content
I realize that I've solved the problem, but I'm still wondering why it doesn't work in the first code? Thanks in advance!
When you are defining $scope.reader
in the first example, you are also calling $scope.reader
(actually $scope.reader.location
) within the definition. At the point you call $scope.reader.location
you have not finished defining $scope.reader
. So it gets confused...
In the second method, you first define the $scope.reader
object, then add a new property ( scanning
) to it, setting the value based in part on $scope.reader
. This is fine since the $scope.reader
object now exists
At the point in which you construct the object literal for $scope.reader
, you haven't actually defined $scope.reader
yet — you're trying to reference a variable that does not exist yet.
However, by setting it afterwards, in your new code, $scope.reader
has been set, so you can reference it without any issues.
This is javascript problem, not really angularjs. location
doesn't exist until the following line $scope.reader = {};
is finished.
It is something along this line:
var A = 100;
var B = A = B;
//What is the value of B?
//Does (B = A) execute first, so B = 100?
//Does (A = B) execute first, so B = undefined?
//answer, B is undefined.
//this is analogy to your case
// var R.A = 100 never happens until R = {} is finished.
// so B = A is undefined
var R = {
A: 100,
B: A
};
The expression is evaluated prior to assignment. You statement is like this:
var temp = {
location: 500,
state: "q1",
scanning: $scope.cellArray[$scope.reader.location].content
};
$scope.reader = temp;
So when the property is being assigned a value, $scope.reader
has not yet been set.
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