Function.prototype.toString ( )

When the toString method is called with an object func as its thisValue, the following steps are taken:

1. If func is a Bound Function exotic object, then return an implementation-dependent string source code representation of func. The representation must conform to the rules below. It is implementation dependent whether the representation includes bound function information or information about the target function.
2. If Type(func) is Object and is either a built-in function object or has an [[ECMAScriptCode]] internal slot, then return an implementation-dependent string source code representation of func. The representation must conform to the rules below.
3. Throw a TypeError exception.

toString Representation Requirements:

• If func was defined using ECMAScript code,

  • If func was not defined using MethodDefinition or GeneratorMethod, then the string representation must be such that if the string is evaluated using eval in a lexical context that is equivalent to the lexical context used to create func, an object will be created such that the object's [[Call]] internal method is indistinguishable from the [[Call]] internal method of func. Two [[Call]] internal methods are indistinguishable if, for all thisArgument values, argumentList values, and current execution context stack context, each method returns the same value according to SameValue and has the same effects on context when invoked with thisArgument and argumentList in the execution context context. The string representation must not mention freely any variables that were not mentioned freely by func's source code, even if these "extra" names were originally in scope. NOTE: This rule can be widened to include MethodDefinition and GeneratorMethod by saying the [[Call]] behaviour would be the same if the function does not contain (according to the Contains static semantic rule) this, new.target, or super.
  • If func was defined using FunctionExpression or FunctionDeclaration, the string representation must have the syntax of a FunctionExpression.
  • If func was defined using ClassExpression or ClassDeclaration, the string representation must have the syntax of a ClassExpression.
  • If func was defined using GeneratorExpression or GeneratorDeclaration, the string representation must have the syntax of a GeneratorExpression.
  • If func was defined using ArrowFunction, the string representation must have the syntax of an ArrowFunction.
  • If func was defined using GeneratorMethod, the string representation must have the syntax of a GeneratorExpression. Else, if func was defined using MethodDefinition, the string representation must have the syntax of a FunctionExpression.
  • NOTE: I would like to add an assertion about preserving arity.

• Else, if func has an [[ECMAScriptCode]] internal slot, the string representation must have the syntax of a FunctionExpression. NOTE Functions created using the Function constructor are not defined using ECMAScript code, but do have an [[ECMAScriptCode]] internal slot.

• Else, the string representation must have the syntax of a NativeFunction.

NativeFunction : function BindingIdentifier_opt ( FormalParameters ) { [ native code ] }