Conditional variable existence, returning error when nonexistent

Question

I have a function that has the parameter _parameter. Ideally, I would like it to be able to contain one or many different JSON-formatted values, without requiring them all to be present. I have constructed a series of conditions to check this, and below is one snippet from the set, as this (and its archetype) is throwing the error below. How do I check to see if _parameters["location"]["area"] exists without throwing an error and without using the try...catch as that would result in tremendous redundancy?

What I would like to use is the following code, or some variation as long as it is still a ternary statement:

this.area = typeof _parameters["location"]["area"] !== 'undefined' ? _parameters["location"]["area"] : this.location["area"];

For giggles, I tried these, as well:

this.area = _parameters["location"]["area"] !== undefined ? _parameters["location"]["area"] : this.location["area"];

if(_parameters["location"]["area"]) {
    alert(true);
} else {
    alert(false);
}

However, these both return the following error and no alert() menu is ever seen:

"Uncaught TypeError: Cannot read property 'area' of undefined"

This throws the same error, but resolves gracefully:

try {
   _parameters["location"]["area"]; 
} catch (e) {
    alert(e);
}

EDIT For clarity, here are some options that _parameter could contain, and an idea of the structure that I'm trying to produce:

this.name = _parameters["name"] !== undefined ? _parameters["name"] : this.name;
this.gender = _parameters["gender"] !== undefined ? _parameters["gender"] : this.gender;
this.location = {
    area : typeof _parameters["location"]["area"] !== 'undefined' ? _parameters["location"]["area"] : this.location["area"],
    x : _parameters["location"]["x"] !== undefined ? _parameters["location"]["x"] : this.location["x"],
    y : _parameters["location"]["y"] !== undefined ? _parameters["location"]["y"] : this.location["y"]
};

Andrew Cooper · Accepted Answer · 2014-05-24 00:36:18Z

2

You need to check for location before you check for area. Try this:

this.area = _parameters["location"] && _parameters["location"]["area"] ? _parameters["location"]["area"] : this.location["area"];

You should also be able to use the dot notation for your property access:

this.area = _parameters.location && _parameters.location.area ? _parameters.location.area : this.location.area;

answered May 24, 2014 at 0:36

Andrew Cooper

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2 Comments

Matt Over a year ago

Is there any functional reason to prefer a dot notation over the bracketed notation?

Andrew Cooper Over a year ago

They're semantically equivalent. The dot notation has less syntactic noise, and is more idiomatic JavaScript. The array notation is useful if the property name has a hyphen or other character that would otherwise be illegal, but otherwise the dot notation tends to be easier to read.

Greg · Accepted Answer · 2014-05-24 00:35:25Z

2

Should be able to prefix it with a simple check to see if _parameters["location"] is set.

this.area = (_parameters["location"] && typeof _parameters["location"]["area"] !== 'undefined') ? _parameters["location"]["area"] : this.location["area"];

Although this could be shortened to:

this.area = (_parameters["location"] && _parameters["location"]["area"]) ? _parameters["location"]["area"] : this.location["area"];

Since undefined properties evaluate to false in conditionals.

answered May 24, 2014 at 0:35

Greg

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