如果可以，请用 defineProperty ：

  var obj = {
 _a：1 
}; 

 Object.defineProperty（obj，a，{
 get：function（）{
 return this._a; 
}，

 set：function（newA）{
 if（this.changeB）{
 clearTimeout（this.changeB）; 
 this.changeB = null; 
} 

 if（this.a == 0&& newA == 1）{
 this.changeB = setTimeout（function（）{
 this.b = 1; 
 } .bind（this），500）; 
} 
 else if（this.a == 1&& newA == 0）{
 this.b = 0; 
} 

 this._a = newA; 
} 
}）; 
 

然后，您可以像这样使用它：

  //立即设置为0 
 obj.a = 0; 
 console.log（obj.b）; 

 //设置为1并开始超时
 obj.a = 1; 
 console.log（obj.b）; 
 setTimeout（function（）{
 console.log（obj.b）; 

 //设置回0 
 obj.a = 0; 
 console.log（obj.b）; 

 //嘿，确保更改止损b设置为
 obj.a = 1; 
 obj.a = 2; 
 setTimeout（function（）{
 console.log（obj.b）; 
}，500）; 
}，500）; 
 

I have a javascript variable a and a variable b that can have a value of 0 or 1.

Can anyone suggest how I could code a function sob could depend on the value of a like this:

• When a changes from 0 to 1 - if a is 1 for more than 500ms then b is set to 1
• When a changes from 1 to 0 - b is set to 0 immediately

If there's a way to code this using a function then could that be attached to the variable a's setter ?

If you can, wrap the access with defineProperty:

var obj = {
    _a: 1
};

Object.defineProperty(obj, "a", {
    get: function() {
        return this._a;
    },

    set: function(newA) {
        if (this.changeB) {
            clearTimeout(this.changeB);
            this.changeB = null;
        }

        if (this.a == 0 && newA == 1) {
            this.changeB = setTimeout(function() {
                this.b = 1;
            }.bind(this), 500);
        }
        else if (this.a == 1 && newA == 0) {
            this.b = 0;
        }

        this._a = newA;
    }
});

Then, you can use it like so:

// Immediately set to 0
obj.a = 0;
console.log(obj.b);

// Set to 1 and start the timeout
obj.a = 1;
console.log(obj.b);
setTimeout(function() {
    console.log(obj.b);

    // Set back to 0
    obj.a = 0;
    console.log(obj.b);

    // And hey, make sure changing a stops b from being set
    obj.a = 1;
    obj.a = 2;
    setTimeout(function() {
        console.log(obj.b);
    }, 500);
}, 500);