What is the type of “keys” in JavaScript?

Question

I bumbed into one of those moments when I just lose the focus and start wondering on a silly question:

var a = {
  b: \"value\"
}

What is t

Answer 1

Property names are automatically coerced into a string. You can try this yourself by using a numeric literal as a property name.

var object = {
  .12e3: 'wut'
};
object[.12e3]; // 'wut'
object['.12e3']; // undefined
object['120']; // 'wut'

// Let’s try another numeric literal:
object = {
  12e34: 'heh'
};
object[12e34]; // 'heh'
object['12e34']; // undefined
object[1.2e35]; // 'heh'
object['1.2e35']; // undefined
object[1.2e+35]; // 'heh'
object['1.2e+35']; // 'heh'

For this reason, I’d recommend using only string literals for property names.

From Unquoted property names / object keys in JavaScript, my write-up on the subject:

Quotes can only be omitted if the property name is a numeric literal or a valid identifier name.

[…]

Bracket notation can safely be used for all property names.

[…]

Dot notation can only be used when the property name is a valid identifier name.

I also made a tool that will tell you if any given property name can be used without quotes and/or with dot notation. Try it at mothereff.in/js-properties.