Javascript Object with default value

Question

What I want to achieve seems quite simple but I'm not sure if it's possible.

I'd like to have an object that returns a certain value if no property is specified. For example:

console.log(obj) // Returns "123" console.log(obj.x) // Returns "ABC"

you can't do it for console.log(obj), but you can do it for console.log(obj + '') — zb'
– zb', Commented Mar 23, 2016 at 2:54
@Bergi Misuse of that code will end in tears. I stand by your first answer. — Wainage
– Wainage, Commented Mar 23, 2016 at 2:59

Bergi · Accepted Answer · 2016-03-23 03:37:08Z

1

Override the toString() method in the prototype for your custom object.

function MyObj() { } MyObj.prototype.toString = function () { return '123'; }; var obj = new MyObj(); obj.x = 'ABC'; console.log(obj + ''); console.log(obj.x + '');

edited Mar 23, 2016 at 3:37

Bergi

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answered Mar 23, 2016 at 3:10

Dave Anderson

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1 Comment

Dave Anderson Over a year ago

As @zb points out this only works by coercing the object into a string with + '' in the log call.

Dmytro Omelyan · Accepted Answer · 2020-02-28 12:14:53Z

Here's how it can be done using Symbol's toPrimitive:

const primaryColor = { default: 'green', darker: 'silver', lighter: 'white', } Object.defineProperty(primaryColor, Symbol.toPrimitive, { value: () => primaryColor.default });

so, we got something like:

console.log('primary color: ' + primaryColor.darker) // returns "primary color: silver" console.log('primary color: ' + primaryColor) // returns "primary color: green"

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