extends ___ = ___ syntax in typescript -


i saw syntax here

export interface actionreducer<t, v extends action = action> {   (state: t | undefined, action: v): t; }

i confused syntax <t,v extends action = action>

i not find documentation syntax in typescript . can point me there . looks new syntax build not able recognize , gives me error : 

node_modules/@ngrx/store/src/models.d.ts:6:52: ',' expected.

generic parameter defaults

checked versions 1 , figured out added in version 2.3 specifies default type of v . v can either class extends action or if not mentioned of type action.

doc


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