Question

First time working with javascript and JSON. Curious why this causes a comiler error:

var dataTypes = new function() {
    this.list = "list"; this.boolean = "boolean";
};  

var jsonDataTypes = [
        {dataTypes.list:"Food For Lunch"}
];

When this doesn’t.

var dataTypes = new function() {
    this.list = "list"; this.boolean = "boolean";
};  

var jsonDataTypes = [
        {"Food For Lunch":dataTypes.list}
];

Why am I allowed to use a variable for the value but not for the key?

The error is:

Multiple markers at this line
    - Missing semicolon
    - Syntax error on token(s), misplaced 
     construct(s)
    - Missing semicolon
    - Syntax error on tokens, delete these 
     tokens

Answer 1

First, you are working with JavaScript objects, not JSON. Objects are a data type in JavaScript, whereas JSON is a data transfer format.

The keys inside object literals must be valid identifiers, strings or numbers, because keys are interpreted literally. Identifiers are not allowed to have dots (.) and certain other characters in it. See the specification for more information.

As you want to use the value of dataTypes.list as key, you have to create the object in two steps:

var jsonDataTypes = [{}];
jsonDataTypes[0][dataTypes.list] = "Food For Lunch";

or if you want to use it literally, use a string:

var jsonDataTypes = [
    {"dataTypes.list": "Food For Lunch"}
];