expression_if, expression_unless

This feature is available since Karabiner-Elements 15.5.19.

Change an event when the evaluated result of the expression is true/false.

Example

Send Command+Q only when it has been pressed twice.

{
    "description": "Prevent unintended Command+Q presses by ignoring it unless it's double-pressed",
    "manipulators": [
        {
            "type": "basic",
            "from": {
                "key_code": "q",
                "modifiers": {
                    "mandatory": ["command"],
                    "optional": ["any"]
                }
            },
            "to": [
                {
                    "key_code": "q",
                    "modifiers": ["command"]
                }
            ],
            "conditions": [
                {
                    "type": "expression_if",
                    "expression": "command_q_expiration > system.now.milliseconds"
                }
            ]
        },
        {
            "type": "basic",
            "from": {
                "key_code": "q",
                "modifiers": {
                    "mandatory": ["command"],
                    "optional": ["any"]
                }
            },
            "to": [
                {
                    "set_variable": {
                        "name": "command_q_expiration",
                        "expression": "system.now.milliseconds + 300"
                    }
                }
            ]
        }
    ]
}

Specification

Tip

expression_if and expression_unless are designed to be used with the following features:

{
    "type": "expression_if",
    "expression": expression
}

Name	Required	Description
`type`	Required	`"expression_if"` or `"expression_unless"`.
`expression`	Required	Target expression.

Expression specification

expression allows you to write arithmetic expressions, and you can use variables set by set_variable manipulations and the following system-provided variables. If an undefined variable appears in the expression, its value is treated as 0.

system.now.milliseconds
system.scroll_direction_is_natural
system.use_fkeys_as_standard_function_keys

The arithmetic syntax used in expression follows exprtk.

Confirm the current variable values

You can see the current variable values by EventViewer > Variables.