difference between dot notation and bracket notation in javascript [duplicate]

Question

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JavaScript property access: dot notation vs. brackets? (17 answers)

Closed 5 years ago.

I am trying to understand the difference between .Notation and [] notation. In my problem below when I use if (object[key] === true) I get the correct answer. When I use if (object.key === true) it does not work. Could someone explain why it's different.

var myObj = {
    one: false,
    two: false,
    three: false,
    four: true,
    five: false,
    six: false
};
var myFunc = function (object) {
    for (var key in object) {
        if (object[key] === true) {
            return "There is a true value in this object";
        } else {

        }
    }
    return "Sorry, there are no true values in this object";
};

Answer 1

When you use dot notation, key means the actual property in the object, which will not be there. So, undefined is returned which is not equal to true.

When you use [] notation you are accessing the property in the object with the name in the variable key. So, that will work.

For example,

var myObj = {
    myVar : 1
};

for (var key in myObj) {
    console.log(key);
    console.log(myObj.key);
    console.log(myObj[key]);
}

This will print,

myVar
undefined
1

Because, myObj has no member named key (myObj.key tries to get the member with the name key) and in the next case, myObj has a member named myVar (myObj[key] tries to get the member with the value in key).

Dot Notation

jslint prefers dot notation.

[] Notation

This offers flexibility. You can dynamically access the members with a variable.

Answer 2

Dot notation is faster to write and clearer to read.

Square bracket notation allows access to properties containing special characters and selection of properties using variables.

<form id="myForm">
<div><label>
<input type="checkbox" name="foo[]" value="1"> 1
</label></div>
<div><label>
<input type="checkbox" name="foo[]" value="2"> 2
</label></div>
<div><label>
<input type="checkbox" name="foo[]" value="3"> 3
</label></div>
</form>

Example with errors:

var inputs = myForm.foo[];

Square bracket notation, on the other hand, allows:

var inputs = myForm["foo[]"];

Since the square brackets are part of a string, their special meaning doesn't apply.The second advantage of square bracket notation is when dealing with variable property names.

for (var i = 0; i < 10; i++) {
doSomething(myForm["myControlNumber" + i]);
}