Check permissions in Sharepoint based on a PermMask

In some cases you could get this kind of hexadecimal numbers from Sharepoint for the PermMask: 0xb008431061, or 0x1b03c4313ff, and so on.

To verify the PermMask against the different level of permissions you can proceed with the below method (using functions from core.js):

var permMask = '0x1b03c4313ff';
var permissions = new SP.BasePermissions();
permissions.initPropertiesFromJson({High:GetPermMaskH(permMask), Low:GetPermMaskL(permMask)});
// we can now check permissions using SP.BasePermissions.has()
// and we can compare with SP.PermissionKind — see https://msdn.microsoft.com/en-us/library/office/ee556747(v=office.14).aspx
var canEdit = permissions.has(SP.PermissionKind.editListItems);

On Sharepoint 2010 GetPermMaskH and GetPermMaskL are not defined, so here is their code:

function GetPermMaskH(b) {
    var a = b.length;
    return a <= 10 ? 0 : parseInt(b.substring(2, a - 8), 16)
}

function GetPermMaskL(b) {
    var a = b.length;
    return a <= 10 ? parseInt(b) : parseInt(b.substring(a - 8, a), 16)
}

It took me many hours to find how to proceed, so I hope it will help some others.