Exploring Functions in Object Literal Notation in TypeScript: Why is the Context of 'this' Assigned as Type 'any'?

It appears that the question has been posed before, but my search yielded no results. The issue at hand seems rather straightforward. TypeScript integrates well with object literal notation, however, when methods are defined within, it struggles to handle the this context inside those functions. It appears that this is assigned the type any, resulting in a lack of auto-completion.

var foo = {
    log(str) {
        console.log(str);
    },
    print(str) {
        this. /* No autocompletion */
    }
};    
foo. /* Normal autocompletion */

Playground

One would naturally expect that auto-completion should function properly within a method just like it does when utilizing the foo variable. While it's possible to first declare a class and then instantiate the object, why is it necessary to do so without declaring an intermediate class or interface? In this simple case, it feels unnecessary like boilerplate code.

Answer №1

The Official TypeScript Documentation states that when it comes to functions:

"[...] is still any. This is because this is derived from the function expression within the object literal."

For more detailed explanations about the this keyword and its context, check out the GitHub TypeScript Wiki.

Answer №2

Just a heads up for future use: Given [email protected], this behavior may seem a bit intricate:

  • If the method explicitly defines a this parameter, then this will have the type of that parameter.

  • Alternatively, if the method is contextually typed by a signature with a this parameter, then this will take on the type of that parameter.

  • However, if --noImplicitThis is turned on and the containing object literal contains a contextual type that includes ThisType, then this will have type T.
  • On the other hand, if --noImplicitThis is enabled and the containing object literal does not include ThisType in its contextual type, this will adopt the contextual type.
  • Or, when --noImplicitThis is enabled but there are no specific conditions met, this will simply inherit the type of the containing object literal.
  • Lastly, if none of the above apply, this will default to type any.

source

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