Typescript - Pushing values from one array to another outputs integers

Question

I've come across some strange behavior that I cannot find the root cause of when pushing the contents of one string array to another.

I have one array that I first empty to make room for the new array's values and then I iterate over the new array, pushing its contents to the first emptied array. Here is how the code looks:

oldArray.length = 0;
console.log(newArray);
for (let item in newArray) {
  console.log(item);
  this.oldArray.push(item);
}

The output of the first print is:

["a string", "another string", "a third string"]

The output of each print inside the for loop is:

0, 1, 2

This is where I am lost, I am for some reason getting the indexes of the items instead of the strings themselves. Could it be related to the way in which I am emptying the old array? Note that for this purpose, it's important that I do not mess up the oldArray reference, which I'd prefer to keep intact.

Evert · Accepted Answer · 2018-05-26 21:35:52Z

4

When you use for( .. in ..), you iterate over the indexes of the array, not the values themselves. To get the value, you could do this:

console.log(newArray[item]);

It's even better to just use the for(.. of ..) loop instead.

for (const item of newArray) {

Aside: use const instead of let for this case.

answered May 26, 2018 at 21:35

Evert

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1 Comment

Måns Thörnvik Over a year ago

That's exactly it! I was very confused since it seemingly worked in another place, but there I was using 'of' and not 'in'...

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