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JavaScript JavaScript Loops, Arrays and Objects Tracking Data Using Objects Mixing and Matching Arrays and Objects

Posted by mebula
mebula
4,931 Points

Dot vs Bracket Notation

Why does Dave use dot notation when mixing objects/arrays, but when he was explaining For In loops, he specifically said to use bracket notation or else the function will be looking for a property/key with the exact name used in the dot notation.

Can somebody please explain the difference?

Thanks!

1 Answer

andren
andren
28,558 Points
andren
andren
28,558 Points

Well the difference is pretty much exacly what you noted in your question. When you use dot notation JavaScript will look for properties whose name match the word you provide exactly.

With bracket notation JavaScript will interpret your input as code, meaning that if you reference a variable name for instance then JavaScript will look for properties matching the value of that variable, not the name of the variable itself.

Here is an example:

var exampleString = "message"

var exampleObject = {
    message: "Hello World"
}

console.log(exampleObject[exampleString])
// JavaScript will look for a property called "message"
// since that is what the "exampleString" holds.

console.log(exampleObject.exampleString)
// JavaScript will look for a property called "exampleString"
// since that is the exact word you provided.

As you can probably guess the first of those console.log statements ends up printing out "Hello World". While the second ends up printing undefined, since exampleString is not a property that exist in the exampleObject.