TypeScript: Array Assignment

Question

I have a TypeScript class with an object property. The properties inside this property have the type Array. I expected to be able to add instances of my FlowConnection class to these arrays like I would in JavaScript, but the following code produces a compiler error:

export class FlowComponent{
    protected connectionPoints = {
        input: Array<FlowConnection>(),
        output: Array<FlowConnection>()
    }

    addInput(newInput:FlowConnection):Array<FlowConnection>{
        var l = this.connectionPoints.input.length;
        return this.connectionPoints.input[l] = newInput;
}

The specific compiler error occurs on line 9 of the code above, and is as follows:

error TS2322: Type 'FlowConnection' is not assignable to type 'FlowConnection[]'.

Trying to use Array.push instead of assigning to the index at the end of the array yields an even stranger result:

return this.connectionPoints.input.push(newInput);

error TS2322: Type 'number' is not assignable to type 'FlowConnection[]'.

What am I missing here?

Ben Seymour · Accepted Answer · 2016-08-08 15:21:51Z

2

return this.connectionPoints.input[l] = newInput; doesn't return an instance of an Array - nor does return this.connectionPoints.input.push(newInput); - do the push, then return!

this.connectionPoints.input.push(newInput);
return this.connectionPoints.input;

For reference:

return this.connectionPoints.input[l] = newInput; //returns newInput
return this.connectionPoints.input.push(newInput); //returns new array length

edited Aug 8, 2016 at 15:21

Ben Seymour

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answered Jul 18, 2016 at 20:05

tymeJV

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4 Comments

Ben Seymour Over a year ago

Thanks! The first compiler error makes much more sense now. For the second, I assume that Array.push returns the index that the new item was pushed to? (Easy enough to test for myself, so don't feel compelled to answer unless you want to :P )

tymeJV Over a year ago

@B1SeeMore -- I believe Array.push returns the new length of the array, not the index

Ben Seymour Over a year ago

Well, I feel especially bright today. :P No shortage of sites hosting JavaScript references - I should have just gone and looked it up. Thanks for answering!

tymeJV Over a year ago

@B1SeeMore -- It's always the easy ones that get ya :D

ChrisoLosoph · Accepted Answer · 2023-07-09 11:22:38Z

A concrete answer would be, the return value does not match your specified return type. The returned value is of type FlowConnection (when returning the assignment) but it expects Array<FlowConnection>.

Your example also misses a closing brace (curly bracket) to close the addInput function.

Theoretically, a modern more convenient type system could be designed where scalar types are considered to be constrained collection types with only one element. Unfortunately, TypeScript's type system doesn't go that far.

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