回调函数无法访问父函数范围内的变量

Question

In the below code, variable text is not accessible in the findTextToDelete function (it generates an error)

 array = ['a', 'b', 'removeThis', 'c']; removeText("removeThis"); function removeText(text) { array.splice(array.findIndex(findTextToDelete),1); } function findTextToDelete(element) { return element === text; } 

I am able to get around this by creating a global variable 'globalText':

 var globalText = ""; array = ['a', 'b', 'removeThis', 'c']; removeText("removeThis"); function removeText(text) { globalText = text; array.splice(array.findIndex(findTextToDelete),1); } function findTextToDelete(element) { return element === globalText; } console.log(array) 

However I am trying to understand why the first method does not work.

It seems there must be a better way to do this. Is there a way to pass 'text' into the callback function?

Any help would be appreciated.

Answer 1

You could use a closure over text for finding the element.

 function removeText(text) { array.splice(array.findIndex(findTextToDelete(text)), 1); } function findTextToDelete(text) { return function (element) { return element === text; } } var array = ['a', 'b', 'removeThis', 'c']; removeText("removeThis"); console.log(array) 

Answer 2

Yes, I think you are confused with closure .

Callback function doesn't have caller's scope. So you can't access the variable text in findTextToDelete .

If you want to access the variable, pass it to the callback function as an argument.

Answer 3

Locals defined in one function are only available to other functions that are defined within the first function. The scope of locals defined in a function is limited to that function. This is generally known as lexical scoping . (Other languages have different rules, but lexical is common in imperative languages.)

So you need to define a function inside removeText to access text .

One way is to just use a function literal rather than a named function:

function removeText(text) {
    globalText = text;
    array.splice(array.findIndex(function (element) {
        return element === globalText;
    }, 1);
}

Another way is to forward the extra value:

function removeText(text) {
    globalText = text;
    array.splice(array.findIndex(function (el) { returnfindTextToDelete(el, text); }, 1);
}

function findTextToDelete(element, text) {
    return element === globalText;
}

(If you're not stuck with backward compatibility then arrow functions make both easier to write.)