Count the number of true members in an array of boolean values

问题

New to javascript and I'm having trouble counting the number of trues in an array of boolean values. I'm trying to use the reduce() function. Can someone tell me what I'm doing wrong?

   //trying to count the number of true in an array
    myCount = [false,false,true,false,true].reduce(function(a,b){
      return b?a++:a;
    },0);
    alert("myCount ="+ myCount);  // this is always 0

回答1:

Seems like your problem is solved already, but there are plenty of easier methods to do it.

Array.prototype.filter() - the easiest one, in my opinion.

console.log([true,false,true,false,true].filter(v => v).length);

or even simplier:

console.log([true,false,true,false,true].filter(Boolean).length);

Array.prototype.forEach()

var myCounter = 0;

[true,false,true,false,true].forEach(v => v ? myCounter++ : v);

console.log(myCounter);

回答2:

You're returning a++ when the value is true, which will always be zero. Post-increment happens after the value is retrieved. So on the first iteration, a is 0, and the value of a++ is also 0, even though a is incremented. Because a and b are parameters of the callback, it's a fresh a on each call.

Instead:

myCount = [false,false,true,false,true].reduce(function(a,b){
  return b?a+1:a;
},0);

回答3:

You should use ++a instead a++ because you have to change the value of a suddenly. a variable will be incremented after its value is returned.

 myCount = [false,false,true,false,true].reduce(function(a,b){
      return b? ++a:a;
    },0);
alert("myCount ="+ myCount);

回答4:

I am not really sure about this, but a and b aren't numbers.

You shoud do something like :

//trying to count the number of true in an array
myCount = [false,false,true,false,true].reduce(function(a,b){
cpt = 0;
if(a) cpt++;
if(b) cpt++;
return cpt;
},0);
alert("myCount ="+ myCount);  // this is always 0

来源：https://stackoverflow.com/questions/42317140/count-the-number-of-true-members-in-an-array-of-boolean-values

标签

javascript

arrays

ternary-operator

reduce