Simple way to return default value If accessing properties of possibly undefined values

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If accessing properties of undefined value, I'm getting an exception:

let object = {}
let n = object["foo"].length;

VM186:1 Uncaught TypeError: Cannot read property 'length' of undefined at :1:12

I want to get a default value in this case instead of an exception, but the way I'm doing it now seems too verbose:

let n = 0;
if (object.hasOwnProperty("foo")) {
    n = object["foo"].length;

Is there a more simple and elegant way to do this? Possibly, using ES6.

Not sure if more elegant, but object destructuring can assign default values. It wont prevent you from null values though. Just undefined

const obj = {};
const { foo: { length = 0 } = [] } = obj;

undefined, A quick workaround is using a try/catch helper function with ES6 arrow function: function getSafe(fn, defaultVal) { try { return fn(); } catch (e) { return defaultVal; } } // use it like this getSafe(() => obj. This is actually possible to do with Object.create.It will not work for "non defined" properties. But for the ones that has been given a default value. var defaults = { a: 'test1', b: 'test2' };

using es6 you can check whether any key is there for the object by checking using Object.keys(object) which will give an array of keys Object.keys, checking it with the length will give that object is empty or not and also checking one more condition whether the constructor of object is an Object.

Please see the below code. if those two conditions are satisfied which means object is empty and you can assign a default value

let object = {}
if(Object.keys(object).length === 0 && object.constructor === Object){
// assign a default value if the object is empty
 object["foo"] = "bar"

console.log("object is empty default value will be assigned", object)

How to avoid 'cannot read property of undefined' errors?, 7 tips on how to handle correctly 'undefined' and increase code durability. to the uninitialized state and accessing undefined is simply not possible. When accessing a non-existing object property, JavaScript returns undefined . And set a default value (an empty object { } ) for the config parameter,  Destructuring Basics: By destructuring an object, you can choose properties from an object by key name. You can define as few or as many of the properties you like, and default values are set by a basic syntax of let {key = default} = object.

Try ternary operator:

let n =object["foo"] ? object["foo"].length : 0;

To not write twice object["foo"] you can also do something like that:

let n = object["foo"]; 
n = n ? n.length : 0;

What is Default Values - Database?, When you use destructuring, make sure to set a default value of empty `{}` to prevent it This is because you can't destructure undefined and null values function hi (person) { return typeof person; } hi(); // undefined It's a library for accessing arbitrarily nested, possibly nullable properties on a How the empty {} is useful. If it was undefined you could use a default argument for your logger and avoid null checking for property access. Again, this is probably a corner case that is easy to handle, so I'm not recommending it as a supported use case per se, just trying to illustrate what some users might be trying.

We have two possible undefined values to check. First, the key needs to exist on dictionary. After that, the object should be an array or a struct with the property length.

One great way to go is to use || to define a default value when undefined. But it will not work if you tried to check length of an undefined value, without dealing with this first.


   let object = {};
   let r = (object["foo"] || 333);
   let l = r.length || 111;

   let oneLiner = (object["foo"] || 333).length || 111;

   console.log("Default Value: " + r);
   console.log("Default Length: " + l);
   console.log("OneLiner: " + oneLiner);

How to check if a JavaScript object property is undefined, Furthermore, the use of different keywords for properties can affect both the format In memory Ondisk on disk Simple Internal format Storage format Transient Transient properties are particularly useful when you need to store temporary values case they remain undefined or—when present—assigned a default value. A default value expression produces the default value of a type. There are two kinds of default value expressions: the default operator call and a default literal. You also use the default keyword as the default case label within a switch statement.

The method mentioned by @lleaon will work only when the value is undefined and won't work for other falsy values like null.

Here is a technique I use often to safely access nested objects in JavaScript. I picked it up a year ago from another SO answer.

const obj = {};
const arrLength = ( || []) || 0;
console.log(arrLength); // 0

You can check deep nest level like this,

const obj = {};
const arrLength = ((obj.nestedObj || {}).foo || []) || 0;
console.log(arrLength); // 0

In case you're iterested, I wrote a blog post on it a while back.

Safely Accessing Nested Objects in JavaScript

7 Tips to Handle undefined in JavaScript, It has a name property, the default value of which is "Error", and a message errors when numeric values exceed the defined bounds of possible numeric values. invalid references, such as accessing the properties of an undefined variable. ECMAScript 5, the toString method of error objects is required to return strings  MATLAB assigns the specified default values to properties only once when MATLAB loads the class definition. If you use a handle class constructor to create a property default value, MATLAB calls the constructor only when the class is first used, and then uses the same object handle as the default for the property in all objects created.

Prevent Error with Default {} when Destructuring, In JavaScript, function parameters default to undefined . values in the function body and assign a value if they are undefined . function multiply(a, b) { return a * b } multiply(5, 2) // 10 multiply(5) // NaN ! because the default parameter value does not have access to the child scope of the function body: `Property cannot be accessed on possibly undefined value` when access to object's key of map #5047 Closed sottar opened this issue Oct 6, 2017 · 4 comments

Object-Oriented Application Development Using the Caché , When used with function calls, it returns undefined if the given This results in shorter and simpler expressions when accessing chained properties when the a way to simplify accessing values through connected objects when it's be used after optional chaining in order to build a default value when  That is why my code above always verifies that the value is undefined before I attempt to apply default values. Just keep in mind the onInit gets called often. You want the code to be as fast and optimized as possible. For example, make sure the value you want to set as default is really empty before you call an API to get a default value

Pro JavaScript with MooTools, If you've been working with JavaScript for any length of time, you've probably run Sometimes you can solve this by merging with a default object, but at other times an undefined/null property // then stop executing and return undefined. if (obj safe way to do deep property access and even get back a useful value when  JavaScript is designed on a simple object-based paradigm. An object is a collection of properties, and a property is an association between a name (or key) and a value. A property's value can be a function, in which case the property is known as a method. In addition to objects that are predefined in the browser, you can define your own objects. This chapter describes how to use objects