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rated 0 times [ 182] [ 1] / answers: 1 / hits: 166643 / 10 Years ago, mon, march 31, 2014, 12:00:00

I'm having difficulties understanding how the track by expression of ng-repeat in angularjs works. The documentation is very scarce: http://docs.angularjs.org/api/ng/directive/ngRepeat

Can you explain what the difference between those two snippets of code is in terms of databinding and other relevant aspects?

with: track by $index

<!--names is an array-->

<div ng-repeat=(key, value) in names track by $index>

  <input ng-model=value[key]>                         

</div>

without (same output)

<!--names is an array-->

<div ng-repeat=(key, value) in names>

   <input ng-model=value[key]>                         

</div>

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1 Year ago

answered 10 Years ago malkajillc · Accepted Answer

You can track by $index if your data source has duplicate identifiers

e.g.: $scope.dataSource: [{id:1,name:'one'}, {id:1,name:'one too'}, {id:2,name:'two'}]

You can't iterate this collection while using 'id' as identifier (duplicate id:1).

WON'T WORK:

<element ng-repeat=item.id as item.name for item in dataSource>

  // something with item ...

</element>

but you can, if using track by $index:

<element ng-repeat=item in dataSource track by $index>

  // something with item ...

</element>