在异步javascript调用中设置变量[重复]

Question

我有一个 x 类 C 的对象。属性x.myval 应通过调用x.load() 来设置，而x.load() 又应从异步ajax 调用中获取其数据。

class C {
    constructor() {
        this.myval = "hello";
    }
    load() {
        return $.ajax({
            type: "GET",
            url: "file.txt",
            // suppress "xml not well-formed error in firefox", 
            beforeSend: function(xhr){
                if (xhr.overrideMimeType) { xhr.overrideMimeType("text/plain"); }
            },
            contentType: "text/plain",
            dataType: "text",
            success: function(text) {
                this.myval = text;
            }
        });
    }
}

var x = new C();
$.when(x.load()).done(function(a1,a2) {
    console.log(x.text); //should print the content of file.txt
});

我收到错误this.myval is undefined，显然是因为this 设置为jquery-$ 的this。

我也试过了：

class C {
    constructor() {
        this.myval = "hello";
    }
    load() {
        var callback = function callbackClosure(mythis) {return function(text) {
            this.myval = text;
        }}(this);

        return $.ajax({
            type: "GET",
            url: "file.txt",
            // suppress "xml not well-formed error in firefox", 
            beforeSend: function(xhr){
                if (xhr.overrideMimeType) { xhr.overrideMimeType("text/plain"); }
            },
            contentType: "text/plain",
            dataType: "text",
            success: callback
        });
    }
}

var x = new C();
$.when(x.load()).done(function(a1,a2) {
    console.log(x.text); //should print the content of file.txt
});

但这导致了一个例外jQuery.Deferred exception: assignment to undeclared variable...

Answer 1

this 在这一行中没有引用您的实例：

success: function(text) {
    this.myval = text;
}

您可以在外部方法中创建一个引用您的实例的临时变量，如下所示：

load() {
    var that = this;
    return $.ajax({
        type: "GET",
        url: "file.txt",
        // suppress "xml not well-formed error in firefox", 
        beforeSend: function(xhr){
            if (xhr.overrideMimeType) { xhr.overrideMimeType("text/plain"); }
        },
        contentType: "text/plain",
        dataType: "text",
        success: function(text) {
            that.myval = text;
        }
    });
}

或者，您可以使用箭头函数：

load() {
    return $.ajax({
        type: "GET",
        url: "file.txt",
        // suppress "xml not well-formed error in firefox", 
        beforeSend: function(xhr){
            if (xhr.overrideMimeType) { xhr.overrideMimeType("text/plain"); }
        },
        contentType: "text/plain",
        dataType: "text",
        success: (text) => {
            this.myval = text;
        }
    });
}