in javascript, when constructing an object, unable to use other objects' methods

Question

when constructing an object using methods from other objects as an attribute name get Syntax Error: Unexpected token . - cannot find correct syntax

var R = function(a) { this.arg = a; };
R.prototype.name = function() { return this.arg; }

var r1 = new R('abc');
var name1 = r1.name();        // => "abc"

var o1 = { 'abc': r1 }        // this works with constant
var o2 = { name1: r1 }        // does not work with variable (see answer)
var o3 = { r1.name(): r1 }    // this fails - syntax
var o4 = { 'abc': r1.name() } // this works

have tried { (r1.name()): r1 }, but that fails as well.

please note that strings and integers are evaluated as barewords whereas methods and variables are not:

var o5 = { e.1: 123 }          // fails
var o6 = { 'e.1': 123 }        // succeeds
var o7 = { 1: 123 }            // succeeds
var o8 = { '1': 123 }          // same as o7

possible duplicate of How to use a variable value for the key of another object? — Felix Kling
– Felix Kling, Commented Aug 8, 2011 at 13:12

cdhowie · Accepted Answer · 2011-08-08 13:06:13Z

3

var o2 = { name1: r1 }        // this works with variable

This has the same meaning as:

var o2 = { 'name1': r1 }

In other words, it's still treating name1 as a literal string. The key in an object literal must be a constant or a so-called "bareword" -- this is a restriction of the language. Variables and function invocations cannot be used as keys in object literals (but they can be used as values). Using variables will appear to work, but the variable name will be used as the key and not the variable's value.

You will have to do something like:

var o2 = {};
o2[name1] = r1;

var o3 = {};
o3[r1.name()] = r1;

answered Aug 8, 2011 at 13:06

cdhowie

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3 Comments

cc young Over a year ago

ok - this makes sense. I guess it goes without asking there is no bareword function. :-) sometimes this language just loves for you to keyboard

cdhowie Over a year ago

What would such a function do? A bareword is a language element, not some kind of special value.

cc young Over a year ago

I understand - lame joke on my part

ek_ny · Accepted Answer · 2011-08-08 13:05:44Z

0

var o4 = { 'abc', r1.name() }

this one should be:

var o4 = { 'abc': r1.name() }

answered Aug 8, 2011 at 13:05

ek_ny

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2 Comments

cc young Over a year ago

right! problem is on the attribute name side. will edit description above

ek_ny Over a year ago

i think cdhowie has the answer-- no dynamic keys in object literal syntax.

vol7ron · Accepted Answer · 2011-08-08 14:41:18Z

0

You could create your own property setter

function setProperty (prop,val,obj) {
    if (typeof prop === "undefined" || typeof val === "undefined") return undefined;
    if (typeof obj === "undefined") obj = {};
    obj[prop] = val;
    return obj;
}

// use 1
var o1 = setProperty(r1.name(),r1);

// use 2
var o2 = {};
setProperty(r1.name(),r1,o2);

answered Aug 8, 2011 at 14:41

vol7ron

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2 Comments

cc young Over a year ago

good idea, but I think if I were doing this I would write a createObject with arguments arg0=name0, arg1=value1, .... to return an object.

vol7ron Over a year ago

you're using the base object type {} which is different from Object or even x = function (){}. All the above is doing is setting a property, first creating it if it doesn't exist.

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