Javascript callback function get parent variable value

Question

function foo(callback){
    var a = 1;
    
    return callback();
}

foo(function(){ 
    if ( a == 1 ) { alert(a); }
});

I try to make a callback can access parent var, but I get undefine

anyone know how to achieve this

How much research effort is expected of Stack Overflow users? — Andreas
– Andreas, Commented Jun 11, 2021 at 14:41
It can't, that's why you can simulate private state through closures in JS. You can pass it's (current) value to the callback (or in the case of objects, a reference) but not the variable; which means, the callback doesn't "notice" when the value of a changes; all it has is the value given. And you can not change the value of the variable from the callback. If you explain what exactly you are trying to achieve, we can help you work around the problems. — Thomas
– Thomas, Commented Jun 11, 2021 at 14:55

canon · Accepted Answer · 2021-06-11 14:37:52Z

4

Make a an argument and pass it to the callback:

function foo(callback){
    var a = 1;
    
    return callback(a);
}

foo(function(a){ 
    if ( a == 1 ) { alert(a); }
});

answered Jun 11, 2021 at 14:37

canon

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Comments

Rush W. · Accepted Answer · 2021-06-11 14:38:17Z

1

Try sending variable "a" in callback function as parameter.

function foo(callback){
    var a = 1;
    
    return callback(a);
}

foo(function(a){ 
    if ( a == 1 ) { alert(a); }
});

answered Jun 11, 2021 at 14:38

Rush W.

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