group and increment a value in array of object in js

Question

i have an array of objects:

arr=[
{a: \'xm\', b: 1},
{a: \'dm\', b: 1},
{a: \'xm\', b: 2},
{a: \'xm\', b: 1},
{a: \'dm\', b: 4},
{a: \'xm\', b: 1},
{a: \'dm\'