How to use the undefined keyword In Typescript
We know that the JavaScript objects returns an undefined if a key does not exist. Being a superset of JavaScript, TypeScript also returns an undefined if a key does not exist. Therefore, we can perform a simple check for undefined to determine the existence of a key in a dictionary in TypeScript.
Syntax
if(dictionary[key] !== undefined){
// Key exists
} else{
// Key does not exist
}
Example: The below code uses the ‘undefined’ keyword to check if a key exists in dictionary in TypeScript.
interface Dictionary{
[key:string]:string;
}
let dictionary:Dictionary = {
"name": "w3wiki",
"desc": "A computer science portal",
"est": "2009" };
let keyToCheck:string = "subjects";
if (dictionary[keyToCheck] !== undefined) {
console.log(
`Key ${keyToCheck} exists in the dictionary.`);
} else {
console.log(
`Key ${keyToCheck} doesn't exist in the dictionary.`);
}
Output:
"Key subjects doesn't exist in the dictionary."
How to Check if a Key Exists in a Dictionary in TypeScript ?
In TypeScript dictionaries are used whenever the data is needed to be stored in key and value form. We often retrieve the data from the dictionaries using an associated key. Therefore it becomes crucial to check whether the key exists in a dictionary or not.
We can use the below methods to check if a key exists or not in a dictionary.
Table of Content
- Using an ‘in’ operator
- Using the hasOwnProperty method
- Using the undefined keyword
- Using the Map object
- Using for loop
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