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Accessing property value in an object.

I am studying 'object literals' in javascript. I am a biginner.
If object property name is using valid identifier, its value can be accessed as 1).
Otherwise, it can be accessed like 2) using "[]"
Since 2 is not valid object property name in 3), I understand foo.2 causes an error. However, it says that 'missing ) after argument list'
Can you please explain that?
Also, I don't understand why I am getting an error for 4)
And 5) and 6) are valid statements.
What's the difference between foo[a] and foo["a"]?
what's the difference between foo[2] and foo["2"]?

var foo = {a: "alpha", 2: "two"};
console.log(foo.a);    // alpha ---------1)
console.log(foo[2]);   // two ---------2)
//console.log(foo.2);  // Error: missing ) after argument list -----------3)
//console.log(foo[a]); // Error: a is not defined-----------------4)
console.log(foo["a"]); // alpha --------------5)
console.log(foo["2"]); // two -------------6)

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1 Solution
console.log(foo.2);  // Error: missing ) after argument list -----------3)
the interpreter think you miss something to have a valid statement, this is for it an right parenthesis

//console.log(foo[a]); // Error: a is not defined-----------------4)
the right syntax is to put simple or double quotes around "a":
console.log(foo["a"]); // or console.log(foo["a"]);
or you must define << a >> as any other variable, for example :
var a = 2;
console.log(foo[a]);   // two ---------2)
var a = "a";
console.log(foo[a]);    // alpha ---------1)

sometime you want to use a key as variable
in foo[a] , a is a variable
in foo["a"], "a" is a key

it's not comparable to {a:1234}
we usualy don't add the quotes, as the interepreter know << a >>, the member on the left side of << : >> gonna be always a key
you can't use a variable to define a key using {}, you gonna use :
var foo = {};
var var1 = "a";
var var2 = "2"; // or 2
foo[var1] = "alpha";
foo[var2] = 'two';
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