Answer by peterhurford for Detect 64-bit or 32-bit Windows from User Agent or Javascript?

For 64-bit Windows with 64-bit IE window.navigator.platform will be "Win64" and window.navigator.cpuClass will be "x64".

For 64-bit Windows with 32-bit IE, window.navigator.platform will be "Win32" and window.navigator.cpuClass will be "x86".

For 32-bit Windows, window.navigator.platform will be "Win32" and window.navigator.cpuClass will be undefined (I think).

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Source: I made an app that uses JavaScript to determine if someone is using a 32 bit or 64 bit processor. You can see the code here on GitHub.