Is there an __repr__ equivalent for javascript?

Question

The closest I got to something close to Python's repr is this:

function User(name, password){          this.name = name;          this.password = password; } User.prototype.toString = function(){     return this.name; };    var user = new User('example', 'password');  console.log(user.toString()) // but user.name would be even shorter 

Is there a way to represent an object as a string by default? Or am I going to have to just use object.variable to get the results I want?

Answer 1

JSON.stringify is probably the closest you are going to get from native libraries. It doesn't work well with objects, but you could define your own code to work around that. I searched for libraries that provide this functionality but didn't find anything.

Answer 2

Node.js util.inspect

http://nodejs.org/api/util.html#util_util_inspect_object_options

To get the object representation that appears on the terminal as a string you can do:

const util = require('util'); console.log(util.inspect({ a: "0\n1", b: "c"})); // Equivalent: automatically called by `console.log`. console.log({ a: "0\n1", b: "c"}); 

output:

{ a: '0\n1', b: 'c' } { a: '0\n1', b: 'c' } 

This is the same that would appear on the terminal if you copy pasted the string { a: "0\n1", b: "c"} into the terminal and pressed enter.

Override the inspect method of a class for a custom representation

This was asked at: How to change string representation of objects in Nodejs debug console view and mentioned at: https://stackoverflow.com/a/32866283/895245 and https://stackoverflow.com/a/54667225/895245 but here goes a fuller example:

const util = require('util');  class MyClass {   constructor(a, b) {     this.a = a;     this.b = b;   }   [util.inspect.custom]() {     return `a is ${this.a} and b is ${this.b}`;   } }  const my_object = new MyClass(1, 2); console.log(util.inspect(my_object)); console.log(my_object); 

Output:

a is 1 and b is 2 a is 1 and b is 2 

The default inspect if we hadn't defined a custom [util.inspect.custom] would have been:

MyClass { a: 1, b: 2 } 

That same representation also shows if you just directly inspect the object on a terminal:

> my_object a is 1 and b is 2 

In older Node.js, the syntax to define the custom printer used to be just:

inspect() {   return `a is ${this.a} and b is ${this.b}`; } 

but that was deprecated with message:

[DEP0079] DeprecationWarning: Custom inspection function on Objects via .inspect() is deprecated 

as mentioned at:

• (node:71307) [DEP0079] DeprecationWarning
• https://nodejs.org/api/util.html#util_util_inspect_custom
• https://nodejs.org/api/deprecations.html#deprecations_dep0079_custom_inspection_function_on_objects_via_inspect

Pretty print util.inspect with newlines and indentation

Not possible? Sigh:

• https://github.com/nodejs/node/issues/14638
• Indented, multi-line logging in NodeJS

One big advantage over JSON.stringify is that inspect takes care of circular dependencies for you. But without pretty print, it is a pain.

Tested in Node.js v10.15.1.