Valid property names, property assignment and access in JavaScript

Question

Updated Question

What, exactly, qualifies as a valid property name in Javascript? How do various methods of property assignment differ? And how does the property n

Answer 1

Short Answer

Object property names can be any valid identifier, numeric literal, or string literal (including the empty string).

With that said, there are some potentially confusing intricacies to keep in mind about JavaScript property names, as described below.

And unless you're working with valid (non-negative integer) array indexes, it's a good idea to explicitly assign all numerical property names as strings.

Negative Numbers

What might look like a negative number is actually an expression — something property names do not support.

// SyntaxError
const obj = { -12: 'nope' };

Fortunately, bracket notation handles expressions for us.

// Successful property assignment.
const obj = {};
obj[-12] = 'yup';

Typecasting

All property names are typecasted into strings before being stored.

const obj = {
  12: '12'
};

console.log(typeof Object.keys(obj)[0]); // -> string

Parsing

But even before typecasting occurs, keys are parsed according to the syntax used, and transformed into a decimal literal.

const obj = {
  // Valid string literal
  '022': '022',

  // Interpreted as decimal
  6: '6',

  // Interpreted as floating-point
  .345: '0.345',

  // Interpreted as floating-point
  1.000: '1',

  // Interpreted as floating-point
  8.9890: '8.989',

  // Interpreted as decimal
  000888: '888',

  // Interpreted as octal
  0777: '511',

  // Interpreted as hexadecimal
  0x00111: '273',

  // Interpreted as binary
  0b0011: '3',
};


/* Quoted property name */
console.log(obj['022']); // "022"; as expected
console.log(obj[022]); // undefined; 022 is an octal literal that evaluates to 18 before our lookup ever occurs

/* Valid (non-negative integer) array index */
console.log(obj[6]); // "6"; as expected
console.log(obj['6']); // "6"; as expected

/* Non-valid array index */
console.log(obj[0x00111]); // "273"; we're accessing the property name as it was assigned (before it was parsed and typecasted)
console.log(obj['0x00111']); // undefined; after parsing and typecasting, our property name seems to have disappeared
console.log(obj['273']); // "273"; there it is, we found it using the evaluation of our original assignment