Equivalent of Python’s KeyError exception in JavaScript?

I am trying to access a certain member in a JavaScript object. In order to do this, I need to try out a couple of key values.

For example, Object['text/html'] which will give me an export link for a HTML document. However, not every object of this type will have a text/html key pair value.

In Python I would solve this problem using a Try-Catch block, with the KeyError exception. If I can do something similar in javascript, as in use an exception in a Try-Catch block, that would be great.

However, if alternatives exists instead of try catch blocks, that do achieve the same end goal, I would like to know about them as well.

EDIT:

I would prefer to use an exception over using functions. I do this because the text/html key might not be there, but it should be there. An exception seems more appropriate for this scenario

Answer

Javascript doesn’t generate an exception when reading or writing a property that doesn’t exist. When reading it, it just returns undefined. When writing it, it just creates the property.

You could create your own function that tests to see if the property exists and throws an exception if it does not (but you’d have to call that function whenever), but JS doesn’t make an exception out of that on it’s own like you are asking for.

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If you want to test if a key exists on an object in javascript, you can use this construct with the in operator:

var obj = {};
var key = "test";

if (key in obj) {
    // key exists
} else {
    // key doesn't exist
}

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If you try to read a key that doesn’t exist, you will get undefined as the value.

var obj = {};
var value = obj.test;

alert(value === undefined);

The in operator does a better job of telling you whether the key exists that testing for undefined because undefined is a legal value for a key that exists.

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In many cases, where you control the values that the keys have and a key that is present will never have a falsey value, you can also just check if the key has a truthy value:

var obj = {};
var obj.test = "hello";

if (obj.test) {
    // key exists and has a truthy value
}

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If you want to make sure that the object itself has the property and not any prototype that it is inheriting from, then you can do this:

var obj = {};
var obj.test = "hello";

if (obj.hasOwnProperty(test)) {
    // key exists on the object itself (not only on the prototype)
}