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How to declare nullable type in TypeScript ?
  • Last Updated : 29 Jun, 2020

In vanilla JavaScript, there are two primary data types, null and undefined. Previously in TypeScript, it was not possible to explicitly name these types as “null” and “undefined”. However, it can now be used regardless of the type checking mode.

To assign “undefined” to any property, the –strictNullChecks flag has to be turned off. Keeping this flag on won’t allow to assign “undefined” to members that has no nullable operator. 

The below example represents what happens if the –strictNullChecks flag is enabled and the following code is executed. 

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interface Student {
 name:string;
 age:number;
}
  
let s: Student = {
 name:'Ajay',
 age:null
}

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It will throw the below error:

Type 'null' is not assignable to type 'number'

This error won’t appear if we have –strictNullChecks flag turned off. The strictNullChecks flag protects from referencing nulls or undefined values in the code. It can be enabled by adding the -–strictNullChecks flag as an option to the command-line compiler or adding it to the tsconfig.json file. 

There is a difference between the Nullable type and “optional”. In “optional”, we provide some default value to the member or let it not be considered as a member. However if any member is assigned optional and then still assigned a value as “null” or “undefined”, then it will throw an error.

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interface Student {
 name:string;
 age?:number;
}
  
let s: Student = {
 name:'Ajay',
 age:null
}

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