Use a concatenated (dynamic) string as JavaScript object key? [duplicate]

Question

var test = "test123" var test123 ={     "key" + test: 123 } 

This code doesn't work. What is wrong with "key" + test ?

Answer 1

Because "key" + test is an expression and not an identifier nor a string literal nor a number literal, which are the only things that are allowed as the key in an object literal.

You have to use the [] notation after creating the object for such a dynamic key:

var test123 = {}; test123["key" + test] = 123; 

An identifier is basically the same subset of characters that you can call a variable (letters, numbers, _ and $; may not start with a number), and a string literal is any string enclosed with ' or ".

So, the only types of keys you can use in an object literal are:

{   a0:   true, // valid identifier   $$_:  true, // same   123:  true, // valid numeric literal   012:  true, // same (octal)   0xf:  true, // same (hex)   "@":  true, // not allowed as an identifier   '0a': true  // same } 

Reference: http://es5.github.com/#x11.1.5.

PropertyName :

IdentifierName

StringLiteral

NumericLiteral

Answer 2

With ES6, you can define dynamic keys within an object literal:

const test = "test123" const test123 = { [`key${test}`]: 123 };  //{ keytest123: 123 }