physics.velocity

physics.velocity is a value to read rather than a call: it is written without parentheses.

The API says: {x, y, z} R/W

physics.velocity

function update(dt) {  const velocity = physics.velocity;}

physics.velocity is called with values of the shape its signature asks for. The code sits in update(dt), which the engine calls every frame: dt is the time since the last one, in seconds.