函数接口

为什么类型脚本不警告我正在定义的函数与接口声明不匹配，但是如果我尝试调用该函数，它确实会警告我。

interface IFormatter {
(data: string, toUpper : boolean): string;
};


//Compiler does not flag error here.
var upperCaseFormatter: IFormatter = function (data: string) {
return data.toUpperCase();
}


upperCaseFormatter("test"); //but does flag an error here.
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最佳答案

The interface ensures that all callers of functions that implement the interface supply the required arguments - data and toUpper.

Because TypeScript understands that JavaScript doesn't mind if you pass arguments that aren't used, it cleverly allows this in implementations.

Why is this okay? Because it means you can substitute any implementation of the interface without affecting calling code.

Example: You can substitute either IFormatter implementation and the code works.

interface IFormatter {
(data: string, toUpper: boolean): string;
};


var upperCaseFormatter: IFormatter = function (data: string) {
return data.toUpperCase();
}


var variableCaseFormatter: IFormatter = function (data: string, toUpper: boolean) {
if (toUpper) {
return data.toUpperCase();
}


return data.toLowerCase();
}


// Switch between these at will
//var formatter = upperCaseFormatter;
var formatter = variableCaseFormatter;


formatter("test", true);

If TypeScript didn't do this, your upperCaseFormatter would have to have to have a parameter called toUpper that wasn't used anywhere in the function - which makes the code less readable.


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