Question

我正在尝试为对象创建自定义打印。我注意到在console.log中使用+使用对象的toString。默认toString会返回[object Object]。我还注意到使用,只是打印所有属性及其值。在console.log中使用逗号时，是否可能调用我的自定义print（toString）？

var a = {
    someAProperty: 1,
    toString: function() {
    return "the custom print is " + this.someAProperty;
  }
}

var b = {
    someBProperty: 2
}

class C {
    constructor(someCProperty){
        this.someCProperty=someCProperty;
    };
}

C.prototype.toString = function customPrint(){
    return "someCProperty is "+this.someCProperty;
}

class D {
    constructor(someDProperty){
        this.someDProperty=someDProperty;
    };
}

function printObject(){
    console.log("\nusing , \n a:( object with toString property)",a,"\nb (object without toString):",b,"\nc (class with toString):",new C(1),"\nd (class without toString) ", new D(4));
    console.log("\nusing + \n a:( object with toString property)"+a+"\nb (object without toString):"+b+"\nc (class with toString):"+new C(1)+"\nd (class without toString) "+ new D(4));
}

printObject()

要运行上述代码，我使用了node.js

node print.js

using ,
 a:( object with toString property) { someAProperty: 1, toString: [Function: toString] }
b (object without toString): { someBProperty: 2 }
c (class with toString): C { someCProperty: 1 }
d (class without toString)  D { someDProperty: 4 }

using +
 a:( object with toString property)the custom print is 1
b (object without toString):[object Object]
c (class with toString):someCProperty is 1
d (class without toString) [object Object]

在console.log中使用逗号时，是否可以获得以下输出？即console.log("\nusing , \n a:( object with toString property)",a,"\nb (object without toString):",b,"\nc (class with toString):",new C(1),"\nd (class without toString) ", new D(4));

a:( object with toString property)the custom print is 1
    b (object without toString):[object Object]
    c (class with toString):someCProperty is 1
    d (class without toString) [object Object]

Answer 1

明确致电.toString()：

console.log("\nusing , \n a:( object with toString property)", a.toString(),
    "\nb (object without toString):", b.toString(),
    "\nc (class with toString):", (new C(1)).toString(),
    "\nd (class without toString) ", (new D(4)).toString());

Answer 2

console.log接受可变数量的参数并输出它们。在您的示例中使用+时，您正在执行"some string" + object，因此+最终成为字符串连接（与console.log无关），结果字符串为作为单个参数传递给console.log。当你使用逗号时，你将两个不同的参数传递给console.log，它以自己的方式处理每个参数。

您可以在对象上包含任何内容，除非您强制使用toString，否则会导致其+被调用（例如，使用String(yourObject)或inspect），但某些控制台（例如Node.js中的控制台）如果在您的对象上，则会使用名为const util = require('util'); var obj = { answer: 42 }; console.log("Without inspect", obj); obj[util.inspect.custom] = function() { return "The answer is " + this.answer; }; console.log("With inspect", obj);的函数：

console.log

否则，只需确保在调用{{1}}时强制转换为字符串（可能通过使用包装函数而不是直接调用它）。

在console.log中使用逗号时是否可以调用toString（自定义打印）

2 个答案: