Typescript anonymous array as function parameter

Question

I'm getting this error in Typescript 4.9. I understand why I'm getting the error but I'm not sure how to get around it. I've looked at nullish coalescing but that gives more errors. The parameter will always be a two-dimensional array with one or more sets of data.

private CreatePlots(data: [] ) {
    let valData = data[0][0];
    let plotInfo = <EpmsPlotQueryData>data[0][1];
    GUI.setPopupMsg("Loading query data " + this._processResults + " of " + this._PlotQueryList.length);
    plotInfo.createPlots(valData);
}

The error I'm getting for both references to data;

I can change the function to have data defined as any to get past the errors i.e.

private CreatePlots(data)

but I would like to understand why I can't use an empty array like you can in javascript.

pmoleri · Accepted Answer · 2023-01-17 21:26:38Z

2

If you know the exact type of the parameter, you should declare it like this:

data: Array<[string, EpmsPlotQueryData]> or data: [string, EpmsPlotQueryData][] (both are the same), which is an array of tuples.

private CreatePlots(data: Array<[string, EpmsPlotQueryData]>) {
    let valData = data[0][0];
    let plotInfo = data[0][1];
    GUI.setPopupMsg("Loading query data " + this._processResults + " of " + this._PlotQueryList.length);
    plotInfo.createPlots(valData);
}

As for why you get the error:

data: any[] this is an array of anything
data: [number, string] this is a tuple, you are telling the compiler that data[0] is always number, data[1] is always string, and data[2] doesn't exist.
data: [] this is an empty tuple, you're telling the compiler that you'll get an empty tuple (an array which is always empty).

edited Jan 17, 2023 at 21:26

answered Jan 16, 2023 at 18:14

pmoleri

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4 Comments

MtnManChris Over a year ago

CreatePlots is called after receiving data from a web service call. In this case data[0][0] would be a string and data[0]1] would be a "UserDefinedObject" EpmsPlotQueryData, in General the result from the webservice could be anything. Thank you for clarifying.

pmoleri Over a year ago

Then use Array<[string, EpmsPlotQueryData]> (an array of tuples) @MtnManChris I updated the answer accordingly.

MtnManChris Over a year ago

the answer you supplied of an empty tuple is what I needed. This is so unlike java script where [] is an array of anything. I could specify but the results from the webservice for other calls can be of other object/types so the answer is to not specify a type.

pmoleri Over a year ago

They probably still share some common shape? Like Array<[string, any]>

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