Extend object a with selected properties of object b.
Only properties with defined values are copied.
Parameters
props: string[]
Names of first-level properties to copy over.
a: any
Target object.
b: any
Source object.
allowDeletion: boolean = false
If true, delete property in a if explicitly set to null in b.
Returns any
Argument a.
Remarks
Previous version of this routine implied that multiple source objects could
be used; however, the implementation was wrong. Since multiple (>1)
sources weren't used anywhere in the vis.js code, this has been removed
Extend object a with selected properties of object b. Only properties with defined values are copied.