count string value using map in javascript, return NaN

Question

I am trying to use a map to count duplicate string number in an array, my code:

var map = {};
var myarray = ["John", "John", "John", "Doe", "Doe", "Smith",
               "John", "Doe", "Joe"];

for (var a = 0; a < myarray.length; a++) {
    if (map[myarray[a]] !== null) {
        map[myarray[a]] += 1;
    } else {
        map[myarray[a]] = 1;
    }
}

but when I do console.log(map); it returns

Object {John: NaN, Doe: NaN, Smith: NaN, Joe: NaN}

Why it is NaN? I want to have the result like:

John :4, Doe: 3, Smith: 1, Joe: 1

How can I do? Thank you in advance.

manji · Accepted Answer · 2014-10-28 16:09:53Z

1

map[myarray[a]] !==null will return true when the field is undefined, which is the case when first time encountering a new value.

Should be:

map[myarray[a]] !== undefined

or

map[myarray[a]] != null
// undefined == null

answered Oct 28, 2014 at 16:09

manji

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Comments

dfsq · Accepted Answer · 2014-10-28 16:08:54Z

0

In this case you should not comapre with null strictly === because undefined is not equivalent to null. Try this:

if (map[myarray[a]] != null) {

or simply:

if (map[myarray[a]]) {

answered Oct 28, 2014 at 16:08

dfsq

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1 Comment

Casey Cao Over a year ago

if (map[myarray[a]]) or map[myarray[a]] !== undefined works, thanks a lot:)

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