Iterator objects should not be misused as arrays

• MISUSED_ITERATOR
• Error
• High, Medium
• No tags

This rule applies when an iterator object is misused as an array.

An iterator object provides the next method that returns each element on successive calls. Built-in iterator objects are created in the following places:

1. The return value of generator or async generator function
2. The return value of String.prototype.matchAll()

This rule detects the following misuses of built-in iterator objects:

1. Iterating with for-in loop: In this case, the loop ends immediately because the built-in iterator objects have no enumerable properties. You should use for-of or for-await-of (for async generator) loop instead.
2. Calling array methods: In this case, a TypeError exception is thrown. You should convert the iterator to an array using Array.from() or array spread operator.
3. Accessing the length property: In this case, an undefined value is read because iterators do not have the length property.

Noncompliant Code Example

// Example 1
function *countDown() {
    yield 3;
    yield 2;
    yield 1;
    yield 0;
}
for (let n in countDown()) { // MISUSED_ITERATOR alarm
    console.log(n);
}

// Example 2
const scores = "ID[1]: 90, 80, 100".matchAll(/\d+/g);
scores.shift(); // MISUSED_ITERATOR alarm

// Example 3
const uppers = "Foo Bar Baz".matchAll(/[A-Z]/g);
for (let i = 0; i < uppers.length; i++) { // MISUSED_ITERATOR alarm
  doSomething(i, uppers.next().value);
}

Compliant Code Example

// Example 1
function *countDown() {
    yield 3;
    yield 2;
    yield 1;
    yield 0;
}
for (let n of countDown()) {
    console.log(n);
}

// Example 2
const numberMatches = "ID[1]: 90, 80, 100".matchAll(/\d+/g);
const scores = [...numberMatches];
scores.shift();

// Example 3
const uppers = "Foo Bar Baz".matchAll(/[A-Z]/g);
let i = 0;
for (let letter of uppers) {
  doSomething(i++, letter);
}

Version

This rule was introduced in DeepScan 1.45.0.

See

• Iterators and Generators

• String.prototype.matchAll()

• for…of

• for await…of

• Array.from()

• Spread in array literals