JavaScript & Python: Check if Property is in Object (or Dictionary)

JavaScript

There are more than one method to check it.

First, we can use hasOwnProperty method.

var object = {"one": 2, "tree": "floor", 5: "socks"}; console.log(object.hasOwnProperty("tree")); // will yield true console.log(object.hasOwnProperty(4)); // console output will be false (not in the object)

We'll expect boolean output.

Second, we can use in operator.

var object = {"one": 2, "tree": "floor", 5: "socks"}; console.log("tree" in object); // will yield true console.log(4 in object); // console output will be false (not in the object)

We'll expect boolean output.

Third, straight away access the property name we need to find out.

var object = {"one": 2, "tree": "floor", 5: "socks"}; console.log(object.tree); // will yield "floor" (the value of that property) console.log(object[4]); // console output will be undefined

We'll expect the returned output to be the value of that certain property name. In this method, the returned value undefined can be ambiguous, because we can initialise a property to have an undefined value too. So, um, the first or second method above is recommended.

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Python

In Python, we can use hasattr method or in operator.

dickonary = {"one": 2, "tree": "floor", 5: "socks"} print 1 in dickonary # output will be False print hasattr(dickonary, "one") # output will be True

Using either in operator or hasattr(dictionary, property) method, we'll expect boolean output (True / False).

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Important

For checking either array/list or object/dictionary, make sure the variable instance is not undefined / null in JavaScript, or None in Python.

Therefore, we should put a preliminary test.

JavaScript

var variable_we_need_to_check; // followed by initialisation of that if (typeof(variable_we_need_to_check) === "object") { // check if Array if (variable_we_need_to_check.constructor === Array) { //... then do the array item checking } // check if Object if (variable_we_need_to_check.constructor === Object) { //... then do the object property checking } }

Python

variable_we_need_to_check # followed by initialisation of that if isinstance(variable_we_need_to_check, list): # do item list checking if isinstance(variable_we_need_to_check, dict): # do dictionary property checking

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Well of course, it doesn't have to be strictly like that above typing. It depends on the purpose of that particular checking.

JSLint (JavaScript linter) will surely shows warning if we use "in" for object property checking. You could use the "object property name direct access" method, or use the hasOwnProperty method. Well, because JSLint is about building good structuring habit.