JavaScript and 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.

let object = {"one": 2, "tree": "floor", 5: "socks"}; console.log(object.hasOwnProperty("tree")); // true console.log(object.hasOwnProperty(4)); // false // (Not in the object)

We'll expect boolean output.

Second, we can use in operator.

let object = {"one": 2, "tree": "floor", 5: "socks"}; console.log("tree" in object); // true console.log(4 in object); // false // (Not in the object)

We'll expect boolean output.

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

let object = {"one": 2, "tree": "floor", 5: "socks"}; console.log(object.tree); // "floor" // (Value of that property) console.log(object[4]); // undefined // (Undefined value from that given property name in the object)

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 # False print hasattr(dickonary, "one") # 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

let 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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Surely, it doesn't have to be strictly as above. It depends on the purpose of that particular checking.

For JavaScript, JSLint (JavaScript linter) will show warning if we use in for object property checking. As the substitute, we could use either the object property name direct access method, or the hasOwnProperty method.