TypeScript中对象文字符号内的函数。 为什么`this`上下文是`any`类型的？

Question

Perhaps the question was already asked, but I could not find any. And it seems so naive. TypeScript comes well with object literal notation, but when defining the methods within, than it can't properly handle this context inside that functions, it seems this receives any type, so there is no auto-completion.

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

Playground

Naturally, I would expect that the autocompletion properly works in method, the same way it does when consuming foo variable. I could declare a class first, then instantiate the object, but why it doesn't work without declaring intermediate class or interface , in this simple case it seems like a boilerplate?

Answer 1

According to the official TypeScript Documentation on functions:

[...] is still any . That's because this comes from the function expression inside the object literal.

The TypeScript Wiki on GitHub also has good explanations about the this keyword and its context.

Answer 2

For future reference: Since TS@2.3, this behaviour is a little bit more complicated:

• If the method has an explicitly declared this parameter, this has the type of that parameter.

• Otherwise, if the method is contextually typed by a signature with a this parameter, this has the type of that parameter.

• Otherwise, if --noImplicitThis is enabled and the containing object literal has a contextual type that includes a ThisType, this has type T.
• Otherwise, if --noImplicitThis is enabled and the containing object literal has a contextual type that doesn't include a ThisType, this has the contextual type.
• Otherwise, if --noImplicitThis is enabled this has the type of the containing object literal.
• Otherwise, this has type any.

reference