Get indexOf char inside a string javascript

Question

How can I check the index of a char within a string?

var str = 'alabama';
alert(str.indexOf('a'));

The "indexOf" method seems to work only at the first occurrence of the char. Any ideas?

What are you trying to do? Find all occurrences of 'a'? Count how many 'a's? — arb
– arb, Commented May 8, 2012 at 17:17
I´m trying to get the indexes of the 'a's. In fact, I need to learn the process so I can use it to identify the position of the char and wrap it with a div. — darksoulsong
– darksoulsong, Commented May 8, 2012 at 17:46
If you have to wrap it with a div why don't you use str.replace(/a/g, "<div>a</div>"); ? — user1150525
– user1150525, Commented May 8, 2012 at 17:48

Alnitak · Accepted Answer · 2012-05-08 17:43:37Z

7

To find subsequent occurrences, you need to supply the second parameter to .indexOf, repeatedly starting from one higher than the last occurrence found.

String.prototype.allIndexesOf = function(c, n) {
    var indexes = [];
    n = n || 0;
    while ((n = this.indexOf(c, n)) >= 0) {
        indexes.push(n++);
    }
    return indexes;
}

Test:

> "alabama".allIndexesOf("a")
[0, 2, 4, 6]

EDIT function updated to allow specification of a fromIndex, per String.indexOf.

edited May 8, 2012 at 17:43

answered May 8, 2012 at 17:18

Alnitak

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4 Comments

Alnitak Over a year ago

yeah, there's a bad performance overhead adding to the prototype of built-in types in Chrome - jsperf.com/indexofso/2

Alnitak Over a year ago

Note also that this version preserves the semantics of String.indexOf() - the search string doesn't have to be a single character.

user1150525 Over a year ago

that's true, but the question was just about chars. So charAt will be ok, too.

darksoulsong Over a year ago

Thx, this will do just fine. o/ But I´ll use Bennika´s because I felt a little slowdown in the page using yours (I´m doing this search in a blog post. So its a really huge string).

user1150525 · Accepted Answer · 2012-05-08 17:18:07Z

4

You can write your own function:

function charIndexes(string, char) {
    var i, j, indexes = []; for(i=0,j=string.length;i<j;++i) {
        if(string.charAt(i) === char) {
            indexes.push(i);
        }
    }

    return indexes;
}

answered May 8, 2012 at 17:18

user1150525

Comments

kennebec · Accepted Answer · 2012-05-08 17:54:14Z

0

var str = 'alabama',i=0, ci=[];

while((i=str.indexOf('a',i))!=-1)  ci.push(i++);

alert(ci.join(', '));

answered May 8, 2012 at 17:54

kennebec

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