JavaScript object: access variable property by name as string [duplicate]

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This question already has an answer here:

If I have a javascript object that looks like below

var columns = {
  left: true,
  center : false,
  right : false
}

and I have a function that is passed both the object, and a property name like so

//should return false
var side = read_prop(columns, 'right');

what would the body of read_prop(object, property) look like?

First answer

You don’t need a function for it – simply use the bracket notation:

var side = columns['right'];

This is equal to dot notation, var side = columns.right;, except the fact that right could also come from a variable, function return value, etc., when using bracket notation.

If you NEED a function for it, here it is:

function read_prop(obj, prop) {
    return obj[prop];
}

To answer some of the comments below that aren’t directly related to the original question, nested objects can be referenced through multiple brackets. If you have a nested object like so:

var foo = { a: 1, b: 2, c: {x: 999, y:998, z: 997}};

you can access property x of c as follows:

var cx = foo['c']['x']

If a property is undefined, an attempt to reference it will return undefined (not null or false):

foo['c']['q'] === null
// returns false

foo['c']['q'] === false
// returns false

foo['c']['q'] === undefined
// returns true

Second answer

Since I was helped with my project by the answer above (I asked a duplicate question and was referred here), I am submitting an answer (my test code) for bracket notation when nesting within the var:

<html>
<head>
  <script type="text/javascript">
    function displayFile(whatOption, whatColor) {
      var Test01 = {
        rectangle: {
          red: "RectangleRedFile",
          blue: "RectangleBlueFile"
        },
        square: {
          red: "SquareRedFile",
          blue: "SquareBlueFile"
        }
      };
      var filename = Test01[whatOption][whatColor];
      alert(filename);
    }
  </script>
</head>
<body>
  <p onclick="displayFile('rectangle', 'red')">[ Rec Red ]</p>
  <br/>
  <p onclick="displayFile('square', 'blue')">[ Sq Blue ]</p>
  <br/>
  <p onclick="displayFile('square', 'red')">[ Sq Red ]</p>
</body>
</html>

Third answer

Reprint:https://stackoverflow.com/questions/4255472/javascript-object-access-variable-property-by-name-as-string
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