JavaScript variable scope inside forEach loop

问题

In the code below there is callback function used with forEach loop going over returned results. Is variable 'error' inside forEach loop and 'error' in callback same variables ?

session.getAll(options, function (error, varbinds) {
    varbinds.forEach(function (vb) {
        if (error) 
            console.log('getALL Fail ');
        else              
            console.log(vb.value);            
    });
});

回答1:

Yes, it is the same variable.

I'm not sure how much you know. So, I'm going to explain in detail. Scoping in JavaScript is at the function level*. Think of function definitions as points on a tree. Each point on the tree is a scope. When using a variable, you can only use what is at your current scope and anything available to ancestors going up to the top (global scope). Here are a few rules & examples that may help you better understand:

*UPDATE: ES6 const and let are block-level

Inner functions have access to outer function-level variables

function a() {
    var a = 4;

    function b() {
        alert(a); /* a = 4 */
    }
}

Parameters are defined at the same scope as if they were defined one line below

function a(a) {
    // variable "a" is at same scope as the example above

    function b() {
        alert(a);
    }
}

Variables in adjacent functions are not accessible

Function a() is the parent. b() and c() are its children. Those children cannot access each other's variables.

function a() {

    function b() {
        var aValue = 2;
    }

    function c() {
        alert(aValue); /* "aValue" is undefined here */
    }
}

Location of function definition is what counts

This returns 5 if you run main();:

function getValue(returnFunc) {
    var a = 7;
    alert(returnFunc());
}

function main() {
    var a = 5;
    getValue(function() { return a; }); // anonymous function becomes "returnFunc"
}

Lastly, variable overriding

(function getValue() {
    var a = 5;

    (function () {
        var a = 7;
        alert(a);
    })();

    alert(a);
})();

I tried to avoid using self-invoking functions/IIFEs for these examples but I just couldn't help myself on this last one. It's the easiest way, I think. Run this and you'll get 7, then 5. But, if you exclude "var" on that inner "a"...

(function getValue() {
    var a = 5;

    (function () {
        a = 7;
        alert(a);
    })();

    alert(a);
})();

You'll get 7, 7. This is because "var" creates a new space in memory. Also, if there is a name conflict with something in a higher scope, it gets overridden as a different variable (despite having the same name).

For some more examples, please see: What is the scope of variables in JavaScript?

回答2:

Yes, it's the same variable, it would change if you define another error variable inside the scope of the forEach callback by using the var keyword:

session.getAll(options, function (error, varbinds) {
    varbinds.forEach(function (vb) {
        if (error) //Same error as the error parameter above
            console.log('getALL Fail ');
        else              
            console.log(vb.value);            
    });
});

session.getAll(options, function (error, varbinds) {
    varbinds.forEach(function (vb) {
        var error = false; //New error for this closure.
        if (error) 
            console.log('getALL Fail ');
        else              
            console.log(vb.value);            
    });
});

来源：https://stackoverflow.com/questions/33154718/javascript-variable-scope-inside-foreach-loop

标签

javascript

scope