Setting Javascript Properties Multiple Times

Question

I'm currently learning javascript and I came across this rather odd behavior when setting a property multiple times. For example:

var duck = {feet: 1, feet: 2}

On running

show(duck)

I get

{feet:2}

Is there some weird javascript reason for this behavior? Why is no error thrown?

In which way is this unexpected? You first tell him that feet is 1 and then that it is 2. He'll only remember the last one. — Saturnix
– Saturnix, Commented Jun 20, 2013 at 15:11

bfavaretto · Accepted Answer · 2013-06-20 16:18:45Z

3

You cannot have multiple properties of the same name in an object, but the language lets it pass silently. That's a flaw in the language. ECMAScript 5 strict mode fixed that, so the following will throw an error:

"use strict";
var duck = {feet: 1, feet: 2}
// SyntaxError: Duplicate data property in object literal not allowed in strict mode

edited Jun 20, 2013 at 16:18

answered Jun 20, 2013 at 15:11

bfavaretto

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3 Comments

Surreal Dreams Over a year ago

Is it a flaw, or is it just recognizing a previously set property and overwriting it? Just wondering.

MatthewMartin Over a year ago

I'm racking my brain to think of a scenario where you'd do this on purpose... all I can think of this code generation, maybe to feed eval. If code existed that was doing this on purpose, it might work but it would be too clever for its own good (too hard for subsequent devs to read the intent of the application)

bfavaretto Over a year ago

@SurrealDreams It's both. The flaw is not throwing an error (since duplicate keys in an object literal make no sense).

Katana314 · Accepted Answer · 2013-06-20 15:12:54Z

0

I kind of agree it would make sense to throw an error here. I think that the parser just expands the object declaration into a series of statements, so no part of it is actually invalid.

duck.feet = 1;
duck.feet = 2;

@Blender: I'm guessing show is a method among his libraries that steps through each property and prints it out with its associated value.

answered Jun 20, 2013 at 15:12

Katana314

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Comments

AllTooSir · Accepted Answer · 2013-06-20 15:13:51Z

0

Non-strict JavaScript allows objects to contain multiple properties with the same name. When the same name is used multiple times, only the last declaration is used. Strict mode requires that all property names are unique.

"use strict";

answered Jun 20, 2013 at 15:13

AllTooSir

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