In Typescript, you have multiple options to define a boolean variable such as:
let a = true;
let b: boolean = true;
Given that true is already a boolean value, is it still typical to explicitly declare the variable type (like shown for b)?
If yes, does this practice extend to all primitive types?
Variables in TypeScript are implicitly typed based on their initial assignment (Learn more). For example, both variables below are automatically typed as boolean:
let a = true; // implicitly typed as boolean
let b: boolean = true; // explicitly typed as boolean
If you try to assign a different type to these variables, you will encounter an error:
a = "a string"; // error, type 'string' is not assignable to type 'boolean'
b = "a string"; // error, type 'string' is not assignable to type 'boolean'
There's no strict convention on whether to be explicit or implicit, but it's generally recommended to opt for brevity. For instance, using let a = true; can be clearer than let a: boolean = true;. The value assigned already indicates the type, so repeating it may not be necessary.
Avoiding Implicit 'any' Types
Be cautious of unintentionally creating "implicit any types". This happens when a variable lacks an initial assignment and type specification:
let a; // implicitly typed as `any`
Such variables can accept values of any type:
a = true; // valid
a = "a string"; // valid
To prevent this, specify the type explicitly:
let a: boolean;
a = true; // valid
a = "a string"; // error, type 'string' is not assignable to type 'boolean' -- good!
To completely eliminate accidental implicit 'any' types, consider enabling the compiler option --noImplicitAny, which triggers a compile error when such instances occur. It's advisable to activate this setting.
let a; // compile error with `--noImplicitAny`
When taking the first approach, there is no type defined (for TypeScript), so you have the ability to potentially assign any value to the variable a. It's important to remember that JavaScript is dynamic and as a result, during compilation, there is no check in place to ensure the correct type of the variable.
Consider the following scenario:
let a = true;
a = 'test';
On the other hand, with the second approach, it enables you to verify at compile time whether the type of the variable b aligns perfectly with expectations: for instance, when utilized as an argument for a method or assigned to another property of a variable.
Utilizing the strong typing feature of TypeScript for static checking is advisable. This proactive step helps in identifying potential issues early on during the development phase rather than encountering them at runtime.
For instance:
let a:boolean = true;
a = 'test'; // results in a compilation error
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