TypeError when spreadArray is used on a string containing an emoji/Unicode character

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
javascript, typescript
Domain
tooling

Research direction

Start with tslib.js around line 208 and the __spreadArray entry point, then reproduce the reported TypeError using the provided emoji snippet or CodeSandbox. Check the emitted helper under the relevant tsconfig options; done means spreading an emoji-containing string no longer throws and produces the expected length.

Written by the indexing model from the issue text.

Description

Expected behavior

No error occurs when spreadArray is used on a string containing an emoji/Unicode character.

Current behavior

When spreadArray is used on a string containing an emoji/Unicode character, the following error occurs:

TypeError: Cannot use 'in' operator to search for '0' in ❤️ at __spreadArray

I believe the problem is related to https://github.com/microsoft/tslib/blob/2.3.1/tslib.js#L208 because if you run ...

'0' in '❤️';

// ... or ...

__spreadArray([], "❤️", true);

... the same error occurs.

Steps to reproduce

https://codesandbox.io/s/beautiful-cohen-86fygb?file=/src/index.js:220-251

Context

I need to replace some emojis with images so I am using emoji-regex to find them and get the true length of the emoji so that it can be substituted, they recommend using ...

[...emoji].length

... that under certain tsconfig.json options, will be transpiled to ...

__spreadArray([], emoji, true).length

I have tried other variations, including Array.from() and emoji.length, and only the variation above yields accurate lengths. Fortunately, swallowing the error in a try-catch seems to work so it seems the error is not critical for spreadArray to function for some reason and I do not know why.

Dominant language
TypeScript
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