type for Function.prototype.bind wrongly preserves fields on a function

When calling bind on a function Javascript does not copy the properties across that have been added to that function, however TS treats them as having been copied.

TypeScript Version: 3.7-Beta

Search Terms: function bind

Code

const a = () => { }
a.hello = 'hi'

const b = a.bind({})

b.hello.split("") // Uncaught TypeError: Cannot read property 'split' of undefined

Expected behavior: hello field should not be present on typeof b.

Actual behavior: hello is deemed to exist on typeof b and the program crashes.

Playground Link: http://www.typescriptlang.org/play/?ts=3.7-Beta&ssl=1&ssc=1&pln=7&pc=1#code/MYewdgzgLgBAhjAvDAFASiQPhgbxgXwCg4A6ACwFMAbKkJGAcjIEsHDDRJYAje075mAAmKHPjTtu5arRIQADlWZQUAIlUYA9JpgBVMMDgBXAOZlYAFQCe8igFEATg5AOAXDADCcMGBCwHFHBCMPLOtg5QVowKSlAMMCAAZjBGwhSJghRChEA

Related Issues: https://github.com/Microsoft/TypeScript/issues/212 seems to be where this was discussed and finally implemented, although it’s not directly related.

[ilogico]

BTW, this only happens when using this specific overload.

const f = (a: boolean) => a;
f.value = 42;

const g = f.bind(undefined);
g.value; // OK!

const h = f.bind(undefined, true);
h.value; // Error, as expected

[RichieAHB]

T & was what I was missing, that looks like it would be exactly what was needed 👍