Is it a guarantee that a function called in the head section of a JavaScript for-of loop will only be executed once?

In both Firefox and Chrome, I tested the code snippet below:

function test() {
    console.log("***")
    return [1,2,3]
}

for (const t of test()) {
    console.log(t)
}

After running this code, I noticed that the test function was only called once. This got me thinking - is this the standard behavior for this type of loop? Could it be similar to the initialization part of a regular 

for ([initialization]; [condition]; [final-expression])
loop? To my surprise, neither the MDN documentation nor the ECMA standard provided clear explanations on this topic.

javascriptfor-of-loop

Answer №1

In the ECMAScript specification, the runtime semantics of the for..in and for..of constructs are divided into two parts:

ForInOfStatement : for ( LeftHandSideExpression of AssignmentExpression ) Statement

  1. Let keyResult be ? ForIn/OfHeadEvaluation(« », AssignmentExpression, iterate).
  2. Return ? ForIn/OfBodyEvaluation(LeftHandSideExpression, Statement, keyResult, iterate, assignment, labelSet).

The process involves first evaluating the "head" (including the value to the right of 'of'), followed by the "body" evaluation which uses the key result instead of the original expression.

The details of the "head" evaluation can be found in step 3 of the procedure outlined in this section:

  1. Let exprRef be the result of evaluating expr.

Similarly, the details of the "body" evaluation involve following the iteration process as detailed in step 6 in this section:

  1. Repeat

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