计算JavaScript中多个关联数组中值的出现

Question

What is the most optimal way to count the occurrence of a value among many associative arrays. For instance, we have

var array1 = {
  firstname: 'john',
  lastname: 'bob'
};
var array2 = {
  firstname: 'sara',
 lastname: 'johnson'
};
var array 3 = {
  firstname: 'john',
  lastname: 'paul'
};

how would I in this case count the number of times "john" occurs as a first name? (return value should be 2)

Answer 1

To make a count use the function reduce , you don't need to create additional arrays (using the function filter ) for doing that.

 var array1 = {firstname: 'john',lastname: 'bob'}, array2 = {firstname: 'sara',lastname: 'johnson'}, array3 = {firstname: 'john',lastname: 'paul'}, count = [array1, array2, array3].reduce((a, c) => (a + (Object.keys(c).findIndex(k => k === 'firstname' && c[k] === 'john') > -1)), 0); console.log(count); 

Answer 2

Put them into a single larger array, filter it by the john name, and then check its length:

 const array1 = { firstname: 'john', lastname: 'bob' }; const array2 = { firstname: 'sara', lastname: 'johnson' }; const array3 = { firstname: 'john', lastname: 'paul' }; const input = [array1, array2, array3]; const johnCount = input.filter(({ firstname }) => firstname === 'john').length; console.log(johnCount);