Typescript: Typescript loop showing undefined - angularjs

Question

public Update() {
this.Data = this.Items;
console.log(this.Data);
for (let value of this.Data) {
console.log(value);
}
}
console
[Object, Object, Object]
Object
CandidateName:"B"
ControlId:0
CreatedBy:null
CreationDateTime:null
ExamId1:2000
ExamName:" Safety"
Id:1292353
after last object it showing length:3
when i going to loop over this object,it is throwing error length is undefined,please help me.

Answer 1

If I understand correctly, this.Items is probably undefined in some cases and you cannot iterate.
So:
for (let value of (this.Data || [])) {
This guards against bad values

Answer 2

The for in or for of statement should be avoided for iterating over arrays. It has two "drawbacks":
1) The order is not guarantee
2) Also inherited properties will be listed/enumerated, if enumerable:false is not specified when defining the property.
If for example you add a property to your prototype, this loop will iterate also over that one.
Array.prototype.test = "test";
var a = ['a', 'b'];
for (let i in a) {
console.log(a[i]);
}
for (let i of a) {
console.log(i);
}
You should see also your property printed.
If you change your loop into a sequential for loop:
for (let i = 0; i < this.Data.length; i++ value of this.Data) {
console.log(this.Data[i]);
}
or a:
this.Data.forEach((el) => {
console.log(el);
});
you may not see your issue.

Answer 3

If you want to iterate over objects, You must use Object.keys(your_obj). Because object doesn't have length property. You can iterate through only of type 'Array' or 'String' using for of. You can use Object.keys(your_obj).length for sequential for loop for(var i=0; i<Object.keys(your_obj).length; i++)
public Update() {
this.Data = this.Items;
console.log(this.Data);
for (let obj of this.Data) {
console.log(obj);
//iterate over object here
for(let property of Object.keys(obj)){
// write your logic for specific object
}
}
}
Extension to quirimmo's answer, sequential for loop, use this
this.Data.forEach(function(obj){
console.log(obj);
//iterate over object here
for(var i=0; i<Object.keys(obj).length; i++){
// write your logic for specific object
}
})