Omitting the parameter returns the current value (getter), whereas defining the parameter sets the value (setter) and returns the instance itself for easier chaining.
GreenSock Docs
Timeline
.totalDuration()
[override] Gets or sets the total duration of the timeline in seconds including any repeats or repeatDelays.
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Parameters
value: Number
(default = NaN
)— Omitting the parameter returns the current value (getter), whereas defining the parameter sets the value (setter) and returns the instance itself for easier chaining. Negative values will be interpreted from the END of the animation.
Returns : *

Details
Gets or sets the total duration of the timeline in seconds including any repeats or repeatDelays. duration
, by contrast, does NOT include repeats and repeatDelays. For example, if the tween has a duration
of 10, a repeat
of 1 and a repeatDelay
of 2, the totalDuration
would be 22.
This method serves as both a getter and setter. Omitting the parameter returns the current value (getter), whereas defining the parameter sets the value (setter) and returns the instance itself for easier chaining.
var total = myTimeline.totalDuration(); //gets total duration
myTimeline.totalDuration(10); //sets the total duration