rspec/rules/S6551/javascript/rule.adoc

When calling `toString()` or coercing into a string an object that doesn't implement its own `toString` method, it returns `[object Object]` which is often not what was intended.

== Why is this an issue?

When using an object in a string context, a developer wants to get the string representation of the state of an object, so obtaining `[object Object]` is probably not the intended behaviour and might even denote a bug.

== How to fix it

You can simply define a `toString()` method for the object or class.

=== Code examples

==== Noncompliant code example

[source,javascript,diff-id=1,diff-type=noncompliant]
----
class Foo {};
const foo = new Foo();

foo + ''; // Noncompliant - evaluates to "[object Object]"
`Foo: ${foo}`; // Noncompliant - evaluates to "Foo: [object Object]"
foo.toString(); // Noncompliant - evaluates to "[object Object]"
----

==== Compliant solution

[source,javascript,diff-id=1,diff-type=compliant]
----
class Foo {
  toString() {
    return 'Foo';
  }
}
const foo = new Foo();

foo + '';
`Foo: ${foo}`;
foo.toString();
----

==== Noncompliant code example

[source,javascript,diff-id=2,diff-type=noncompliant]
----
const foo = {};
foo + ''; // Noncompliant - evaluates to "[object Object]"
`Foo: ${foo}`; // Noncompliant - evaluates to "Foo: [object Object]"
foo.toString(); // Noncompliant - evaluates to "[object Object]"
----

==== Compliant solution

[source,javascript,diff-id=2,diff-type=compliant]
----
const foo = {
  toString: () => {
    return 'Foo';
  }
}
foo + '';
`Foo: ${foo}`;
foo.toString();
----

== Resources

=== Documentation

* https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/toString[MDN - ``++Object.prototype.toString()++``]

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