SyntaxError: Invalid shorthand property initializer (V8-based)
SyntaxError: missing : after property id (Firefox)
SyntaxError: Unexpected token '='. Expected a ':' following the property name 'x'. (Safari)
SyntaxError: Unexpected token '+'. Expected an identifier as property name. (Safari)

When creating objects with the object initializer syntax, a colon (:) separates keys and values for the object's properties.

const obj = { propertyKey: "value" };

This code fails, as the equal sign can't be used this way in this object initializer syntax.

const obj = { propertyKey = "value" };
// SyntaxError: missing : after property id

Correct would be to use a colon, or to use square brackets to assign a new property after the object has been created already.

const obj = { propertyKey: "value" };

Or alternatively:

const obj = {};
obj.propertyKey = "value";

If you create a property key from an expression, you need to use square brackets. Otherwise the property name can't be computed:

const obj = { "b"+"ar": "foo" };
// SyntaxError: missing : after property id

Put the expression in square brackets []:

const obj = { ["b" + "ar"]: "foo" };

• Object initializer