merge!

extend.js

a = {a:1,b:2}
b = {b:3,c:4}
$.extend(a, b)
a #=> {a: 1, b: 3, c: 4} 

a = {a:1,b:2}
b = {b:3,c:4}
_.extend(a, b)
a #=> {a: 1, b: 3, c: 4} 

extend2.js

a = {a:1,b:{c:2}}
b = {b:{d: 3}}
$.extend(a, b)
a #=> {a: 1, b: {d: 3}} 

a = {a:1,b:{c:2}}
b = {b:{d: 3}}
_.extend(a, b)
a #=> {a: 1, b: {d: 3}} 


a = {a:1,b:{c:2}}
b = {b:{d: 3}}
$.extend(true, a, b)
a #=> {a: 1, b: {c:2, d: 3}} 

merge!.rb

a = {a:1,b:2}
b = {b:3,c:4}
a.merge! b
a #=> {:a=>1, :b=>3, :c=>4} 

merge.rb

a = {a:1,b:2}
b = {b:3,c:4}
a.merge! b
a #=> {:a=>1, :b=>3, :c=>4}