Typescript Structure array length not working

Question

I have a class with an internal structure that is an array of a structure that I have:

Fields: DataField<any>[] = new Array<DataField<any>>();

I then have a function that adds the structure into the array with the key from the structure used as an index.

AddField(FieldInfo:DataField<any>):void {
    let FieldName:string = FieldInfo.Name;
    this.Fields[FieldName] = FieldInfo;
}

In trying to test that this works, I can see that the structure is created properly, but the length value is not returning correctly. Console.log output:

console.log(TestObject.Fields);
  [ Name: DataField { Name_: 'Name', Size_: 32, Value_: 'My Name' },
    Age: DataField { Name_: 'Age', Size_: 2, Value_: 33 } ]

My test is then:

expect(TestObject.Fields.length).toBe(2);

But this test fails and reports the length is 0.

expect(received).toBe(expected)
Expected value to be (using ===):
  2
Received:
  0

Using the console.log on the TestObject.Fields.length shows 0 as well.

console.log(TestObject.Fields.length) 
  0

this.Fields[FieldName] = FieldInfo, you index a string into an array, do you even array bro? — Trash Can
– Trash Can, Commented Nov 10, 2017 at 5:08

basarat · Accepted Answer · 2017-11-10 05:10:37Z

1

Setting an element by index doesn't change the length:

Fix

Use methods like push slice etc that exist on arrays.

answered Nov 10, 2017 at 5:10

basarat

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2 Comments

Trash Can Over a year ago

The reason x['a'] = 'a' does not change the length is because it just adds a as another property on array instance x, it does not update the underlying contiguous memory block that x points to, arrays are just javascript objects with numeric properties, and javascript allows you to add any properties to existing objects

Steven Scott Over a year ago

@basarat I still want to be able to access the element by the key name as this code is from a library function that is building the array. If I use the push() onto the array, can I always count on the new entry being the last entry, so I can change the index to the key? I think I might be better to move this into a Dictionary object.

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