Typescript: Work around type definition error for javascript builtin

Question

I want to use FormData in typescript. Unfortunately, the generated typescript definition files doesn't support a FormData constructor with a Form Element as detailed in Typescript Issue #1074.

I have the following code:

var formEl = <HTMLFormElement> document.getElementById("myForm");
var formData = new FormData(formEl);

The code gives the following error because the generated definition is wrong:

error TS2346: Supplied parameters do not match any signature of call target.

I want to use the following declaration:

declare var FormData: {
    prototype: FormData;
    new (form?: HTMLFormElement): FormData;
}

But, if I include that type definition, I get this error:

error TS2403: Subsequent variable declarations must have the same type. Variable 'FormData' must be of type '{ new (): FormData; prototype: FormData; }', but here has type '{ new (form?: HTMLFormElement): FormData; prototype: FormData; }'.

How can I work around this issue?

Joe · Accepted Answer · 2014-12-18 05:25:23Z

3

How can I work around this issue?

Potential 1:

Send a PR.

Potential 2:

Update the shipped lib.d.ts in place:

declare var FormData: {
    prototype: FormData;
    new (form?: HTMLFormElement): FormData;
}

Potential 3:

Copy and customize lib.d.ts and compile with --noLib and manually reference your custom lib.d.ts.

Potential 4:

Bypass the type-checker

var formEl = <HTMLFormElement> document.getElementById("myForm");
var formData = new window['FormData'](formEl);

edited Dec 18, 2014 at 5:25

Joe

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answered Dec 17, 2014 at 23:05

basarat

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1 Comment

Joe Over a year ago

Bummer, I was hoping I could just include the new declaration.

Scott Crowder · Accepted Answer · 2017-04-10 16:55:01Z

1

There is a bug in the VS2017 typescript libraries (which may have been fixed in the April 2017 update). You can around the error you noted by turning off the LanguageService in Tools|Options|Text Editor|JavaScript/TypeScript|LanguageService Just uncheck the "Enable the new JavaScript language service." checkbox.

More details of the issue are on https://developercommunity.visualstudio.com/content/problem/25310/unload-projects-hangs-on-close-solution.html

answered Apr 10, 2017 at 16:55

Scott Crowder

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Comments

Daryl · Accepted Answer · 2015-05-18 16:07:43Z

0

There's already a bug on GitHub. It's targeted for fixing in TypeScript 1.6.

Until then, this is a simple workaround in TypeScript:

var formEl = <HTMLFormElement> document.getElementById("myForm");
var formData = <FormData> new (<any> FormData) (formEl);

And the generated javascript, just what you wanted:

var formData = new FormData(formEl);

This should keep working even after they fix the lib.d.ts issue.

answered May 18, 2015 at 16:07

Daryl

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