I want to do the following in javascript but there's something about the syntax I'm not understanding here:
var theObj = { foo: val1, bar: val2 } if ( condition ) { theObj[foo] = newVal return theObj // returns { foo: val1, bar: newVal } } return theObj // returns { foo: val1, bar: val2 } What you have is not an object array, but rather an object literal. Normally, its properties would be accessed as theObj.property, but JavaScript provides an alternative syntax of theObj["property"] when you need to do operations like string manipulation on the property name (like theObj["property_" + numberVar]), or for properties not valid in dot notation (like number properties theObj[12] = "twelve")
If you access the property via [], you would need to quote the string ["foo"], otherwise the parser would be looking for a variable named foo to insert there. However, this simple string property is better accessed with dot notation:
if ( condition ) { theObj.foo = newVal return theObj // returns { foo: val1, bar: newVal } } 
var obj = {1e5: 3}; alert(obj[1e5])