TypeScript 2 beta: undefined lookup on object gives error

[OliverJAsh]

Given:

window[undefined]

I get:

main.ts(21,1): error TS2342: An index expression argument must be of type 'string', 'number', 'symbol', or 'any'.

I don't understand what is wrong about the expression y[undefined]?

[nippur72]

Also, another strange thing is:

var x; 
window[x] = "a";

Type 'string' is not assignable to type 'Window'.

which is a misleading error description

[mhegazy]

@nippur72's example is because Window has an index signature on it. so this seems right to me.

[mhegazy]

For the undefined indexer, this a breaking change, to avoid that, we should allow undefined and null as index expression type if we are not in strict null mode.

[OliverJAsh]

I should have said this only occurs when strict null checking is enabled,
so it's not necessarily a breaking change

On Wed, 13 Jul 2016, 18:28 Mohamed Hegazy, notifications@github.com wrote:

For the undefined indexer, this a breaking change, to avoid that, we
should allow undefined and null as index expression type if we are not in
strict null mode.

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[mhegazy]

the error when using --strictNullChecks is expected. the one that is not is when --strictNullChecks is not on.

[mhegazy]

I should add that the same error is displayed in both cases.

[OliverJAsh]

Oh, I thought I only saw it when I had enabled strict null checking

On Wed, 13 Jul 2016, 18:32 Mohamed Hegazy, notifications@github.com wrote:

I should add that the same error is displayed in both cases.

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[OliverJAsh]

The following only happens when strict null checking is enabled:

const a = 1 as number | undefined;
window[a]; // error

[sandersn]

Fix is available for review at #10009.