Pointer to a global variable in JavaScript

Question

I have vast experience with C, but I'm fairly new using JavaScript. I've been searching for an explanation, but it seems like I'm not phrasing my question right. I need to tell a function which GLOBAL variable it should change. Here's the code:

<!DOCTYPE html>
<html>
 <head>
  <script>
   function test(blah)
   {
    if (!window.blah)
     window.blah = 0;
    window.blah++;
    document.getElementById(blah).innerHTML = window.blah;
   }
  </script>
 </head>
 <body>
  <div id="first">0</div>
  <input type="button" onclick="test('first')" value="change">
  <br>
  <div id="second">0</div>
  <input type="button" onclick="test('second')" value="change">
  <br>
 </body>
</html>

The purpose of the code is to have two separate counters - clicking the first button should increment window.first while clicking the second button should increment window.second. How do I phrase this?

Use window[blah] instead of window.blah

Samuli Hakoniemi
– Samuli Hakoniemi

2014-05-22 11:43:46 +00:00
Commented May 22, 2014 at 11:43 — Samuli Hakoniemi
– Samuli Hakoniemi, Commented May 22, 2014 at 11:43

Pudge601 · Accepted Answer · 2014-05-22 11:48:56Z

2

You need to specify the variable name using the square bracket notation:

function test(blah)
{
    if (!window[blah])
        window[blah] = 0;
    window[blah]++;
    document.getElementById(blah).innerHTML = window[blah];
}

Hopefully this gives an idea of how the dot notation works in comparison to the square bracket notation;

var o = {
    key: 'value',
    foo: 'bar'
};

var key = 'foo';

console.log(o['foo']); // 'bar'
console.log(o[key]);   // 'bar'
console.log(o.key);    // 'value'

edited May 22, 2014 at 11:48

answered May 22, 2014 at 11:43

Pudge601

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3 Comments

Ulrik Over a year ago

Oh, its like an array? Thank you!

plalx Over a year ago

@Ulrik No, it's like a map.

Sebastien C. Over a year ago

@Ulrik In JS, something.blah is the blah member of something, while something[blah] is the member which name is the string in blah.

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