Why 1["foo"] returns undefined instead of an error?

Question

Today I came accross a strange thing in Javascript. When in Chrome console if I execute :

> 1["foo"]

Chrome console returns :

undefined

I was expecting an error though. How is it possible? I fall on that by studying the underscore.js (an old version) invoke method that seems to use that JavaScript property:

 // Invoke a method (with arguments) on every item in a collection.
  _.invoke = function(obj, method) {
    var args = slice.call(arguments, 2);
    var isFunc = _.isFunction(method);
    return _.map(obj, function(value) {
      var func = isFunc ? method : value[method];
      return func == null ? func : func.apply(value, args);
    });
  };

As you can see, value could be a number and if 1["foo"] was raising an error, that code would be unsafe as I could do the following by mistake:

var a = {'foo' : 1}
_.invoke(a, 'foo'}

"Today I came accross a strange thing in Javascript".. This happens to me everyday — Marcus Höglund, Aug 08 '18 at 18:03
You're using "bracket notation" to attempt to access the `foo` property on the `1` object. — Heretic Monkey, Aug 08 '18 at 18:06
For the same reason `1['toString']` will return `ƒ toString() { [native code] }`. All js types are objects that have properties and methods. — Amir Popovich, Aug 08 '18 at 18:10
Have you ever used `(1).toString()` or similar? It's more common in variables, e.g. `let x = 1, y = x.toString();`. Same thing, really. — ASDFGerte, Aug 08 '18 at 18:11
Closer to [Why is 0\[0\] syntactically valid?](https://stackoverflow.com/q/29250950) perhaps... — Heretic Monkey, Aug 08 '18 at 18:12

score 4 · Accepted Answer · answered Aug 08 '18 at 18:06

4

Everything, even primitives, are essentially objects and can have members (properties, methods, etc). All the code in question is doing is attempting to find a member on 1 with the name foo which is not found so undefined is returned.

answered Aug 08 '18 at 18:06

Igor

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Why 1["foo"] returns undefined instead of an error?

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