JS: Property Key

Property key

• Property key must be String type or Symbol type.
• Property value can be any type.

Example. property key of string type

/* property keys of string type  */
console.log({ "a": 1, "b": 2 });
// { a: 1, b: 2 }

Omitting string quotation mark

if the key name doesn't contain space or other odd characters, the quotation mark can be omitted. see JS: Dot vs bracket syntax for property access

// property keys of string type
// omit the quote in string
const jj = { a: 1, b: 2 };
console.log(jj);
// { a: 1, b: 2 }

// verify property key without quote is string type
console.assert(
 typeof Object.keys({ a: 1 })[0] === "string",
);

Implicit conversion of key to string type

If a key is not symbol or string, it is converted to string implicitly.

// number keys are auto converted to string type
console.assert(
 typeof Object.keys({ 3: 9 })[0] === "string",
);

console.assert(
 typeof Object.keys({ 3.2: 9 })[0] === "string",
);

Property key of symbol type

// add a property of symbol key type
const xsym = Symbol();
const jj = { [xsym]: 3 };
console.log(jj);
// { Symbol(): 3 }

• JS: Symbol Tutorial

Computed property key

To evaluate a variable as property key, put it inside a square bracket, like this:

obj = {[expr]:value}

// computed key name from a variable
const xx = "dog";
const xobj = { [xx]: 3 };
console.log(xobj);
// { dog: 3 }

• JS: Object Literal Expression