Data in source code make language service slow

[thorn0]

TypeScript Version: 2.4.1

Code

Add a long array of number literals passed to the Uint8ClampedArray constructor anywhere in the project:

var a = new Uint8ClampedArray([ 0, 0, 0, /* ... 25000 times ... */, 0 ]);

In my case it was a separate file (module), but it doesn't matter.

You can generate the code to paste using this snippet in the browser console:

copy('var a = new Uint8ClampedArray([ ' + '0, '.repeat(25000) + '0 ]);')

Expected behavior:

Having that much data in the code isn't exactly a great idea, but I didn't expect it to slow down the language service that badly. I use VS Code, but it happens on the Playground as well.

Actual behavior:

The autocompletion, tooltips, etc. become really slow. It doesn't have to be the same file where that big array is. On my machine, when I hover any variable anywhere in the project, it takes up to 10 seconds for the tooltip to show up.

However, it works fine this way:

var arr = [ 0, 0, 0, /* ... 25000 times ... */, 0 ];
var a = new Uint8ClampedArray(arr);

Which seems to be a good enough workaround. I only found it after I submitted this issue.

[mhegazy]

checking the file is fairly quick. looks like we are choking in the language service operations.

[ghost]

Should be fixed by #18619