How does `for..of` loop resolve the iterator from an object?

Question

For an object to implement iterable interface it must implement [Symbol.iterator] key that points to a function that returns the iterator. I\'m won

Answer 1

The specific place where the operation is specified is in 7.4.1 GetIterator( obj [ , method ] ). This gets the @@iterator property of the passed object in step 1a. of the abstract operation:

a. Set method to GetMethod(obj, @@iterator).

@@iterator is a well-known symbol that is the Symbol.iterator property on objects.

This is used by for-in and for-of loops due to the productions in 13.7.5.11 Runtime Semantics:

IterationStatement : for(ForDeclaration of AssignmentExpression) Statement

1. Let keyResult be the result of performing ForIn/OfHeadEvaluation(BoundNames of ForDeclaration, AssignmentExpression, iterate).
2. Return ForIn/OfBodyEvaluation(ForDeclaration, Statement, keyResult, iterate, lexicalBinding, labelSet).

Here, you can see the iterator argument passed to ForIn/OfBodyEvaluation is the return value keyResult of ForIn/OfHeadEvaluation. The return value is, in step 7b:

b. Return GetIterator(exprValue).

Thus, for-of loops get the iterator by accessing the @@iterator or Symbol.iterator well-known symbol by specification.