Overwrite a global JavaScript function in TypeScript

Question

In JavaScript, I can overwrite a function:

function testf() {
    console.log('old function')
}
testf = function() {
    console.log("new function");
}

How can I do this in TypeScript? I'm writing a WebP polyfill and would like to overwrite the global createImageBitmap function, but tsc complains:

Cannot assign to 'createImageBitmap' because it is not a variable.

@bugs: I have no idea how I managed to copy the wrong link... I meant this one — musefan
– musefan, Commented Jun 25, 2018 at 7:41
you have to tell your compiler which global variables you are using. I believe there is preset to use browser globals and otherwise write your own d.ts -typing files and declare your globals that tslint can understand them. otherwise typescript is just normal javascript so your code will work even though your editor displays this error. — Jimi Pajala
– Jimi Pajala, Commented Jun 25, 2018 at 7:55
@musefan I'm sorry, I didn't found this question with Google before when I searched for my problem. Should I delete my question? — Fabian Heller
– Fabian Heller, Commented Jun 25, 2018 at 7:56
Don't delete it. At the worst, this one can just be closed as a duplicate — kingdaro
– kingdaro, Commented Jun 25, 2018 at 7:57

kingdaro · Accepted Answer · 2018-06-25 07:56:09Z

0

Doing someName = whatever requires that someName is defined within the current scope as a variable.

You can, however, assign to window (same as assigning the variable of any other object), which should produce the same effect:

window.createImageBitmap = customImageBitmap

Just make sure customImageBitmap has the same signature (argument types and return types) as window.createImageBitmap in order for this to pass the type checker.

answered Jun 25, 2018 at 7:56

kingdaro

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2 Comments

Fabian Heller Over a year ago

The solution is to assign to window (together with the declare function ...). I tried it without window, therefore it didn't worked.

kingdaro Over a year ago

The declare function ... is not needed. See here

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