My first question on here and need help understanding the for in loop in JavaScript.
When I run the following code I get "undefined" from the alert function:
var o = { a: "property 1", b: "property 2", c: "property 3", d: "property 4" } for (p in o) { alert(o.p); } but if I were to change . to [ ] (ie. alert(o[p])) the alert would return the property value as expected.
Why can't I use . to access the object property?
Imagine you have this object:
var o = { a: "property 1", b: "property 2", c: "property 3", d: "property 4", p: "property 5" } and run the following:
for (p in o) { console.log(o.p); } the result will be:
property 5 property 5 property 5 property 5 property 5 Because o.p means that you want to get the value of a property named p.
Similarly in your example, the property p is not defined in your object.
If you want to get the value of a property via a string, you must use [] notation.
o.p means the property "p" of o.
o["p"] means the same thing.
o[x] means some property (whose value is defined by variable x) of o.
o[p] the same.
o[p] is p-th element in array o.
o.p is property p of array o, which is correctly reported as undefined, as it doesn't exist.
If you take a closer look at this for in loop:
var obj = { a: "property 1", b: "property 2", c: "property 3", d: "property 4" } for (var prop in obj) { if (typeof(prop) == 'string') { console.log('This is a string!'); } } You will notice that the what the for in loop is giving you is the name of the property. That's why it would be ok to use obj[prop] because the property would be a string and that is a legal way to access a property of an object.
When you try and execute obj.prop it gives you undefined because property is a string that represents the name of a property, it's not the actual property.
pinstead of a property namedp. Therfore the.operator is specifically for names of property identifiers (whether or not they exist), whereas[]is for using the result of any expression as the property name.