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forInArrays

Reports iterating over an array with a for-in loop.

✅ This rule is included in the ts logical and logicalStrict presets.

A for-in loop (for (const i in o)) iterates over all enumerable properties of an object, including those that are not array indices. This can lead to unexpected behavior when used with arrays, as it may include properties that are not part of the array’s numeric indices. It also returns the index key (i) as a string, which is not the expected numeric type for array indices.

Consider using a construct more suited for arrays, such as a for-of loop (for (const i of o)).

Examples

Section titled “Examples”
const 
const values: string[]
values = ["a", "b", "c"];


for (const 
const key: string
key in 
const values: string[]
values) {
  
var console: Console
The console module provides a simple debugging console that is similar to the
JavaScript console mechanism provided by web browsers.


The module exports two specific components:


    

  • A Console class with methods such as console.log(), console.error() and console.warn() that can be used to write to any Node.js stream.
    • 

  • A global console instance configured to write to process.stdout and
process.stderr. The global console can be used without importing the node:console module.
    • 



Warning: The global console object's methods are neither consistently
synchronous like the browser APIs they resemble, nor are they consistently
asynchronous like all other Node.js streams. See the note on process I/O for
more information.


Example using the global console:


console.log('hello world');
// Prints: hello world, to stdout
console.log('hello %s', 'world');
// Prints: hello world, to stdout
console.error(new Error('Whoops, something bad happened'));
// Prints error message and stack trace to stderr:
//   Error: Whoops, something bad happened
//     at [eval]:5:15
//     at Script.runInThisContext (node:vm:132:18)
//     at Object.runInThisContext (node:vm:309:38)
//     at node:internal/process/execution:77:19
//     at [eval]-wrapper:6:22
//     at evalScript (node:internal/process/execution:76:60)
//     at node:internal/main/eval_string:23:3


const name = 'Will Robinson';
console.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to stderr


Example using the Console class:


const out = getStreamSomehow();
const err = getStreamSomehow();
const myConsole = new console.Console(out, err);


myConsole.log('hello world');
// Prints: hello world, to out
myConsole.log('hello %s', 'world');
// Prints: hello world, to out
myConsole.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to err


const name = 'Will Robinson';
myConsole.warn(`Danger ${name}! Danger!`);
// Prints: Danger Will Robinson! Danger!, to err
@see ― source
console.
Console.log(message?: any, ...optionalParams: any[]): void (+1 overload)
Prints to stdout with newline. Multiple arguments can be passed, with the
first used as the primary message and all additional used as substitution
values similar to printf(3)
(the arguments are all passed to util.format()).


const count = 5;
console.log('count: %d', count);
// Prints: count: 5, to stdout
console.log('count:', count);
// Prints: count: 5, to stdout


See util.format() for more information.
@since ― v0.1.100
log(
const values: string[]
values[
const key: string
key]);
}

Options

Section titled “Options”

This rule is not configurable.

When Not To Use It

Section titled “When Not To Use It”

If your project is a rare one that intentionally loops over string indices of arrays, you can turn off this rule. You might consider using Flint disable comments and/or configuration file disables for those specific situations instead of completely disabling this rule.

Equivalents in Other Linters

Section titled “Equivalents in Other Linters”
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