问题
When I console.log() an object in my JavaScript program, I just see the output [object Object], which is not very helpful in figuring out what object (or even what type of object) it is.
In C# I\'m used to overriding ToString() to be able to customize the debugger representation of an object. Is there anything similar I can do in JavaScript?
回答1:
You can override toString in Javascript as well. See example:
function Foo()
{
}
// toString override added to prototype of Foo class
Foo.prototype.toString = function()
{
return "[object Foo]";
}
var f = new Foo();
alert(f); // popup displays [object Foo]
See this discussion on how to determine object type name in JavaScript.
回答2:
First override toString for your object or the prototype:
var Foo = function(){};
Foo.prototype.toString = function(){return 'Pity the Foo';};
var foo = new Foo();
Then convert to string to see the string representation of the object:
//using JS implicit type conversion
console.log('' + foo);
If you don't like the extra typing, you can create a function that logs string representations of its arguments to the console:
var puts = function(){
var strings = Array.prototype.map.call(arguments, function(obj){
return '' + obj;
});
console.log.apply(console, strings);
};
Usage:
puts(foo) //logs 'Pity the Foo'
puts(foo, [1,2,3], {a: 2}) //logs 'Pity the Foo 1,2,3 [object Object]'
Update
E2015 provides much nicer syntax for this stuff, but you'll have to use a transpiler like Babel:
// override `toString`
class Foo {
toString(){
return 'Pity the Foo';
}
}
const foo = new Foo();
// utility function for printing objects using their `toString` methods
const puts = (...any) => console.log(...any.map(String));
puts(foo); // logs 'Pity the Foo'
回答3:
An easy way to get debuggable output in browser JS is to just serialize the object to JSON. So you could make a call like
console.log ("Blah: " + JSON.stringify(object));
So for an example, alert("Blah! " + JSON.stringify({key: "value"})); produces an alert with the text Blah! {"key":"value"}
回答4:
If you are using Node it might be worth considering util.inspect.
var util = require('util')
const Point = {
x: 1,
y: 2,
[util.inspect.custom]: function(depth) { return `{ #Point ${this.x},${this.y} }` }
}
console.log( Point );
This will yield:
{ #Point 1,2 }
While the version without inspect prints:
{ x: 1, y: 2 }
回答5:
Just override the toString() method.
Simple example:
var x = {foo: 1, bar: true, baz: 'quux'};
x.toString(); // returns "[object Object]"
x.toString = function () {
var s = [];
for (var k in this) {
if (this.hasOwnProperty(k)) s.push(k + ':' + this[k]);
}
return '{' + s.join() + '}';
};
x.toString(); // returns something more useful
It does even better when you define a new type:
function X()
{
this.foo = 1;
this.bar = true;
this.baz = 'quux';
}
X.prototype.toString = /* same function as before */
new X().toString(); // returns "{foo:1,bar:true,baz:quux}"
回答6:
If the object is defined by yourself you can always add a toString override.
//Defined car Object
var car = {
type: "Fiat",
model: 500,
color: "white",
//.toString() Override
toString: function() {
return this.type;
}
};
//Various ways to test .toString() Override
console.log(car.toString());
console.log(car);
alert(car.toString());
alert(car);
//Defined carPlus Object
var carPlus = {
type: "Fiat",
model: 500,
color: "white",
//.toString() Override
toString: function() {
return 'type: ' + this.type + ', model: ' + this.model + ', color: ' + this.color;
}
};
//Various ways to test .toString() Override
console.log(carPlus.toString());
console.log(carPlus);
alert(carPlus.toString());
alert(carPlus);回答7:
With template literals:
class Foo {
toString() {
return 'I am foo';
}
}
const foo = new Foo();
console.log(`${foo}`); // 'I am foo'
回答8:
The Chrome console log allows you to inspect the object.
回答9:
-This operation takes lot of time to complete, and it's use is discouraged according to mozilla docs: https://developer.mozilla.org/es/docs/Web/JavaScript/Referencia/Objetos_globales/Object/proto
-Apparently, modern browsers deprecated .prototype and ECMA6 specifies using proper__proto__ instead.
So for example, if you are defining you own object geoposition you should call __proto__ property instead of .prototype:
var geoposition = {
lat: window.pos.lat,
lng: window.pos.lng
};
geoposition.__proto__.toString = function(){ return "lat: "+this.lat+", lng: "+this.lng }
console.log("Searching nearby donations to: "+geoposition.toString());
回答10:
Here's an example how to stringify a Map object:
Map.prototype.toString = function() {
let result = {};
this.forEach((key, value) => { result[key] = value;});
return JSON.stringify(result);
};
回答11:
You can give any custom objects their own toString methods, or write a general one that you can call on the object you are looking at-
Function.prototype.named= function(ns){
var Rx= /function\s+([^(\s]+)\s*\(/, tem= this.toString().match(Rx) || "";
if(tem) return tem[1];
return 'unnamed constructor'
}
function whatsit(what){
if(what===undefined)return 'undefined';
if(what=== null) return 'null object';
if(what== window) return 'Window object';
if(what.nodeName){
return 'html '+what.nodeName;
}
try{
if(typeof what== 'object'){
return what.constructor.named();
}
}
catch(er){
return 'Error reading Object constructor';
}
var w=typeof what;
return w.charAt(0).toUpperCase()+w.substring(1);
}
回答12:
Rather than overriding toString(), if you include the Prototype JavaScript Library, you can use Object.inspect() to get a much more useful representation.
Most popular frameworks include something similar.
回答13:
You can extend or override in JS
String.prototype.toString = function() {
return this + "..."
}
document.write("Sergio".toString());回答14:
A simple format Date function using Javascript prototype, it can be used for your purpose
https://gist.github.com/cstipkovic/3983879 :
Date.prototype.formatDate = function (format) {
var date = this,
day = date.getDate(),
month = date.getMonth() + 1,
year = date.getFullYear(),
hours = date.getHours(),
minutes = date.getMinutes(),
seconds = date.getSeconds();
if (!format) {
format = "MM/dd/yyyy";
}
format = format.replace("MM", month.toString().replace(/^(\d)$/, '0$1'));
if (format.indexOf("yyyy") > -1) {
format = format.replace("yyyy", year.toString());
} else if (format.indexOf("yy") > -1) {
format = format.replace("yy", year.toString().substr(2, 2));
}
format = format.replace("dd", day.toString().replace(/^(\d)$/, '0$1'));
if (format.indexOf("t") > -1) {
if (hours > 11) {
format = format.replace("t", "pm");
} else {
format = format.replace("t", "am");
}
}
if (format.indexOf("HH") > -1) {
format = format.replace("HH", hours.toString().replace(/^(\d)$/, '0$1'));
}
if (format.indexOf("hh") > -1) {
if (hours > 12) {
hours -= 12;
}
if (hours === 0) {
hours = 12;
}
format = format.replace("hh", hours.toString().replace(/^(\d)$/, '0$1'));
}
if (format.indexOf("mm") > -1) {
format = format.replace("mm", minutes.toString().replace(/^(\d)$/, '0$1'));
}
if (format.indexOf("ss") > -1) {
format = format.replace("ss", seconds.toString().replace(/^(\d)$/, '0$1'));
}
return format;
};
来源：https://stackoverflow.com/questions/6307514/is-it-possible-to-override-javascripts-tostring-function-to-provide-meaningfu