Details on changing to function keys

To send F1-F12 function keys to an application using Karabiner-Elements, there is a bit of a trick. This is because if you simply send an F1 key using Simple Modifications or Complex Modifications, it will be interpreted as media controls (e.g., display_brightness_decrement) by subsequent Function Keys Modifications or the macOS driver.

Therefore, if you want to send F1-F12 keys to an application, you need to send fn+F1-F12 instead.

For example, the following Complex Modifications sends fn+F2 (cell editing) to Excel with command+e.

💡 How to use this example 
{
    "description": "Excel example",
    "manipulators": [
        {
            "type": "basic",
            "from": {
                "key_code": "e",
                "modifiers": {
                    "mandatory": ["command"],
                    "optional": ["caps_lock"]
                }
            },
            "to": [
                {
                    "key_code": "f2",
                    "modifiers": ["fn"]
                }
            ],
            "conditions": [
                {
                    "bundle_identifiers": ["^com\\.microsoft\\.Excel$"],
                    "type": "frontmost_application_if"
                }
            ]
        }
    ]
}

More user-friendly configuration

The above configuration works well only when “Use F1, F2, etc. keys as standard function keys” is disabled in System Settings. If you want it to work in both cases, use variable_if and variable_unless to handle the conditions as shown below.

💡 How to use this example 
{
    "description": "Excel example 2",
    "manipulators": [
        {
            "type": "basic",
            "from": {
                "key_code": "e",
                "modifiers": {
                    "mandatory": ["command"],
                    "optional": ["caps_lock"]
                }
            },
            "to": [
                {
                    "key_code": "f2",
                    "modifiers": ["fn"]
                }
            ],
            "conditions": [
                {
                    "type": "frontmost_application_if",
                    "bundle_identifiers": ["^com\\.microsoft\\.Excel$"]
                },
                {
                    "type": "variable_unless",
                    "name": "system.use_fkeys_as_standard_function_keys",
                    "value": true
                }
            ]
        },
        {
            "type": "basic",
            "from": {
                "key_code": "e",
                "modifiers": {
                    "mandatory": ["command"],
                    "optional": ["caps_lock"]
                }
            },
            "to": [
                {
                    "key_code": "f2"
                }
            ],
            "conditions": [
                {
                    "type": "frontmost_application_if",
                    "bundle_identifiers": ["^com\\.microsoft\\.Excel$"]
                },
                {
                    "type": "variable_if",
                    "name": "system.use_fkeys_as_standard_function_keys",
                    "value": true
                }
            ]
        }
    ]
}

Other examples

Swap f12 and volume_increment in Google Chrome

In practice, swap F12 with fn+F12.

💡 How to use this example 
{
    "description": "Swap f12 in Google Chrome",
    "manipulators": [
        {
            "type": "basic",
            "from": {
                "key_code": "f12",
                "modifiers": {
                    "mandatory": ["fn"],
                    "optional": ["any"]
                }
            },
            "to": [
                {
                    "key_code": "f12"
                }
            ],
            "conditions": [
                {
                    "bundle_identifiers": ["^com\\.google\\.Chrome$"],
                    "type": "frontmost_application_if"
                }
            ]
        },
        {
            "type": "basic",
            "from": {
                "key_code": "f12",
                "modifiers": {
                    "optional": [
                        "caps_lock",
                        "command",
                        "control",
                        "option",
                        "shift"
                    ]
                }
            },
            "to": [
                {
                    "key_code": "f12",
                    "modifiers": ["fn"]
                }
            ],
            "conditions": [
                {
                    "bundle_identifiers": ["^com\\.google\\.Chrome$"],
                    "type": "frontmost_application_if"
                }
            ]
        }
    ]
}