Is setTimeout along with 'this' the most straightforward answer?

Question

Is setTimeout along with 'this' the most straightforward answer?

It is common knowledge that the setTimeout function does not work correctly with this because it executes in a global scope, causing this to refer to the window object.

To test this behavior, I created a simple test:

All you need to do is wrap it within a function:


var o={}
o.a=1;
o.m=function (){alert(this.a);}


setTimeout( 
  function (){
               o.m()  ;
             }

    ,100);

When executed, it properly alerts 1.

I wonder why no one has mentioned this solution before. Is there something I am missing? Does it behave differently?

p.s.: For those interested, here is a demo where this approach fails:


var o={}
o.a=1;
o.m=function (){alert(this.a);}


setTimeout( o.m   ,100); //undefined

javascript

Answer 1

Answer №1

The best approach is to utilize the bind() method to specify the scope of the anonymous function.

var obj = {};
obj.num = 1;

setTimeout(function (){
    alert(this.num);
}.bind(obj), 1000);

Although your solution may yield the same outcome, it fails to access the context of obj within the anonymous function. Instead, it simply invokes a function from the global object obj.

Your use of this.num pertains to the scope of obj.

Answer 2

The best approach is to utilize the bind() method to specify the scope of the anonymous function.

var obj = {};
obj.num = 1;

setTimeout(function (){
    alert(this.num);
}.bind(obj), 1000);

Although your solution may yield the same outcome, it fails to access the context of obj within the anonymous function. Instead, it simply invokes a function from the global object obj.

Your use of this.num pertains to the scope of obj.

Answer 3

Answer №2

One key thing you're overlooking are closures. In JavaScript, your callback function can still access the variable o, even if it wasn't declared globally. This is because the callback was defined within a scope that had access to o.

The reason why JavaScript doesn't garbage collect o is because the callback function retains a reference to it, allowing your code snippet to function correctly.

For instance:

(function() {
    var data = {};
    setTimeout(function() {
        console.log(data);
        console.log(typeof data);
    }, 1000);
}());
console.log(data);

In this example, you'll initially see undefined being logged, followed by {} and object approximately a second later.
Contrast this with:

var obj = { prop: 'value',
            func: function() {
                console.log(this);
            }
};
setTimeout(obj.func, 1000);
console.log(obj.func());

The first log will display the object referred to by obj, but after one second, the same function object will log the global object due to the invocation in the global context. To circumvent this issue, you can use bind or create another closure (though this may be too advanced for now).

I've delved deeper into JavaScript's ad-hoc context binding and reference resolution in discussions found here, here, here, here, and beyond.

Answer 4

One key thing you're overlooking are closures. In JavaScript, your callback function can still access the variable o, even if it wasn't declared globally. This is because the callback was defined within a scope that had access to o.

The reason why JavaScript doesn't garbage collect o is because the callback function retains a reference to it, allowing your code snippet to function correctly.

For instance:

(function() {
    var data = {};
    setTimeout(function() {
        console.log(data);
        console.log(typeof data);
    }, 1000);
}());
console.log(data);

In this example, you'll initially see undefined being logged, followed by {} and object approximately a second later.
Contrast this with:

var obj = { prop: 'value',
            func: function() {
                console.log(this);
            }
};
setTimeout(obj.func, 1000);
console.log(obj.func());

The first log will display the object referred to by obj, but after one second, the same function object will log the global object due to the invocation in the global context. To circumvent this issue, you can use bind or create another closure (though this may be too advanced for now).

I've delved deeper into JavaScript's ad-hoc context binding and reference resolution in discussions found here, here, here, here, and beyond.

Answer 5

Answer №3

In various contexts, the meaning of "this" can vary while you have set "o" as a global definition.

Answer 6

In various contexts, the meaning of "this" can vary while you have set "o" as a global definition.

Is setTimeout along with 'this' the most straightforward answer?

Answer №1

Answer №2

Answer №3

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