Dot and Square Bracket Notation

Question

I'm trying to understand the difference between the Dot and the Square Bracket Notation. While going through various examples here on SO and on some other sites, I came across these two simple examples:

var obj = { "abc" : "hello" };
var x = "abc";
var y = obj[x];
console.log(y); //output - hello

var user = {
  name: "John Doe",
  age: 30
};
var key = prompt("Enter the property to modify","name or age");
var value = prompt("Enter new value for " + key);
user[key] = value;
alert("New " + key + ": " + user[key]);

The first example returns y to be undefined if in third line I replace the obj[x] with obj.x. Why not "hello"

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But in the second example the expression user[key] can simply be replaced with user.key without any anomalous behavior (at-least to me). Now this confuses me as I recently learned that if we want to access properties by name stored in a variable, we use the [ ] Square bracket Notation.

Answer 1

In dot notation, the name after the dot is the name of the property being referenced. So:

var foo = "bar";
var obj = { foo: 1, bar: 2 };

console.log(obj.foo) // = 1, since the "foo" property of obj is 1,
                     //      independent of the variable foo

However, in square-bracket notation, the name of the property being referenced is the value of whatever is in the square brackets:

var foo = "bar";
var obj = { foo: 1, bar: 2 };

console.log(obj[foo])   // = 2, since the value of the variable foo is "bar" and
                        //      the "bar" property of obj is 2

console.log(obj["foo"]) // = 1, since the value of the literal "foo" is "foo" and
                        //      the "foo" property of obj is 1

In other words, dot-notation obj.foo is always equivalent to obj["foo"], while obj[foo] depends on the value of the variable foo.

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In the specific case of your question, note the differences between dot notation and square-bracket notation:

// with dot notation
var obj = { name: "John Doe", age: 30 };
var key = "age";
var value = 60;

obj.key = value; // referencing the literal property "key"
console.log(obj) // = { name: "John Doe", age: 30, key: 60 }


// with square bracket notation
var obj = { name: "John Doe", age: 30 };
var key = "age";
var value = 60;

obj[key] = value; // referencing property by the value of the key variable ("age")
console.log(obj)  // = { name: "John Doe", age: 60 }