javaScript .bind() 不会返回一个新函数，该函数在调用时将具有等于我传递的 arg 的 'this'

Question

这个问题已经有答案了:

Bind more arguments of an already bound function in Javascript (3 个回答)

已关闭 4 年前。

我有以下代码。

var person1 = {
    fname: 'Cristiano',
    lname: 'Ronaldo',
    getName: function() {
        return this.fname + ' ' + this.lname;
    }
}

var person2 = {
    fname: 'Luka',
    lname: 'Modric',
    getName: function() {
        return this.fname + ' ' + this.lname;
    }
}


var logname = function() {
    console.log(this.getName());
}.bind(person1); // not working as expected when .bind() here !!!


var newlogname1 = logname.bind(person1);
newlogname1(); // prints 'Cristiano Ronaldo'

var newlogname2 = logname.bind(person2);

newlogname2(); // prints 'Cristiano Ronaldo'

var person1 = {
        fname: 'Cristiano',
        lname: 'Ronaldo',
        getName: function() {
            return this.fname + ' ' + this.lname;
        }
    }

    var person2 = {
        fname: 'Luka',
        lname: 'Modric',
        getName: function() {
            return this.fname + ' ' + this.lname;
        }
    }


    var logname = function() {
        console.log(this.getName());
    }.bind(person1); // not working as expected when .bind() here !!!


    var newlogname1 = logname.bind(person1);
    newlogname1(); // prints 'Cristiano Ronaldo'

    var newlogname2 = logname.bind(person2);

    newlogname2(); // prints 'Cristiano Ronaldo'

但是如果我更改下面的代码片段，它将按我的预期工作。

var logname = function() {
    console.log(this.getName());
};

var person1 = {
    fname: 'Cristiano',
    lname: 'Ronaldo',
    getName: function() {
        return this.fname + ' ' + this.lname;
    }
}

var person2 = {
    fname: 'Luka',
    lname: 'Modric',
    getName: function() {
        return this.fname + ' ' + this.lname;
    }
}


var logname = function() {
    console.log(this.getName());
};

var newlogname2 = logname.bind(person2);
newlogname2();

var newlogname1 = logname.bind(person1);
newlogname1();

这是什么行为？当我 .bind() 时会发生什么只是在它的声明部分。我不能改变什么this应该意味着当函数运行时，如果我这样做的话。

Answer 1

.bind() 仅在您要绑定(bind)的函数尚未绑定(bind)时才有效:

function f() {
  console.log(this.a)
}

const f_1 = f.bind({a: 1}) // As expected
const f_2 = f.bind({a: 2}) // -Ditto-
const f_2_3 = f_2.bind({a: 3}) // f_2 already bound, can't .bind() to other object

f_1()
f_2()
f_2_3()

/** Check if function is bound, as per @str's comment */
function isBound(fun) {
  return !('prototype' in fun)
}

console.log(
  isBound(f),
  isBound(f_1)
)

更新:添加了检查函数是否已绑定(bind)的方法，按照@str's comment下面