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I have tried using Ukelele and KeyRemap4MacBook to map Fn+WASD (or IJKL) to arrow keys, but didn't find any working solution.

I need them to work with Shift, Cmd and Alt so I can easily select/highlight text with just the left hand.

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up vote 0 down vote accepted

Ukelele or keylayout files can't be used to remap fn, but KeyRemap4MacBook can, at least on my Apple Wireless Keyboard. Try using a private.xml like this:

<?xml version="1.0"?>
    <autogen>__KeyToKey__ KeyCode::A, ModifierFlag::FN, KeyCode::CURSOR_LEFT</autogen>
    <autogen>__KeyToKey__ KeyCode::D, ModifierFlag::FN, KeyCode::CURSOR_RIGHT</autogen>
    <autogen>__KeyToKey__ KeyCode::W, ModifierFlag::FN, KeyCode::CURSOR_UP</autogen>
    <autogen>__KeyToKey__ KeyCode::S, ModifierFlag::FN, KeyCode::CURSOR_DOWN</autogen>

See the source for the key code values and predefined settings.

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This was just the thing I was looking for! Works greatly :). Code seems simple also. I just didn't realise how to use ModifierFlag – OnniMonni Sep 5 '13 at 11:51

Fn is not an OS key. It doesn't return any keycode and is handled by keyboard controller, so you can't remap it

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On Apple keyboards, Fn is key handled by the OS. – Zequez Sep 23 '15 at 4:37

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