accumulator.value()末尾的“未定义”

Question

我遇到了constructor function method this.result中的值错位问题。我不明白为什么我会得到function -undefined结束的结果.

请告诉我，在function中忘记了什么：

function Accumulator(startingValue) {
    this.startingValue = startingValue;

    this.read = function() {
        this.a = +prompt('Your digit: ', '');
    };

    this.value = function() {
        this.value += this.a;
    };

    this.result = function() {
        return this.value + this.startingValue;
    }
}

var accumulator = new Accumulator(1); // starting value 1
accumulator.read(); // sum prompt with current value
accumulator.read(); // sum current prompt with previous prompt and current value
console.log( accumulator.result() ); // display sum result

Answer 1

如果.value应该是一个整数，不要将它定义为一个函数:-)

我认为你应该放弃.value()，.startingValue和.a，到处使用.value。将求和直接放入read方法中。简化为：

function Accumulator(startingValue) {
    this.value = startingValue;

    this.read = function() {
        // the temporary variable might be unnecessary but I left it in for verbosity
        const a = +prompt('Your digit: ', '');
        this.value += a;
    };

    this.result = function() {
        return this.value;
    };
}

var accumulator = new Accumulator(1); // starting value 1
accumulator.read(); // add prompt to current value
accumulator.read(); // add another prompt to current value
console.log( accumulator.result() ); // display sum by calling result() method

您还可能希望在原型上定义方法：

function Accumulator(startingValue) {
    this.value = startingValue;
}
Accumulator.prototype.read = function() {
    this.value += +prompt('Your digit: ', '');
};
Accumulator.prototype.result = function() {
    return this.value;
};

甚至使用现代的class语法，就像@ArtificialBug建议的那样：

class Accumulator {
    constructor(startingValue) {
        this.value = startingValue;
    }
    read() {
        this.value += parseInt(prompt('Your digit: ', ''), 10);
    }
    result() {
        return this.value;
    }
}

Answer 2

有两个问题

this.value = function() {
    this.value += this.a; //this.value is a function
};

和

 console.log( accumulator.value ); // accumulator value is a function which needs to be invoked

搞定

​

function Accumulator(startingValue) {
    this.startingValue = startingValue;

    this.read = function() {
        this.a = (this.a || this.startingValue ) + +prompt('Your digit: ', '');//initialize and add the prompted value
    };

    this.value = function() {
        return this.a; //simply return the value
    };

    this.result = function() {
        return this.a + this.startingValue; //this.a instead of this.value
    }
}

var accumulator = new Accumulator(1); 
accumulator.read(); 
accumulator.read(); 
console.log( accumulator.value() ); // invoke the method

​