Prefer loose array bounds checks in loops

Array bounds checks within loops should verify if a variable is >= or <= the array length instead of exactly equal to the array length. Performing strict bound checks on arrays in loops is brittle and a common cause of subtle bugs.

function handleTasks() {
  const tasks = [
    // ...
  ]
  let currentTaskIndex = 0
 
  do {
    const currentTask = tasks[currentTaskIndex]
 
    // process task
    // ...
 
    currentTaskIndex++
  } while (currentTaskIndex !== tasks.length)
}

This example has two bugs:

if tasks is empty, the first iteration of the while loop will throw an error
the while loop guard is very brittle which is a code smell. if currentTaskIndex somehow gets changed in an unexpected way with future code changes, then the while loop guard could end up going past the end of the tasks array!

An improved version of this code which fixes these buse looks like:

function handleTasks() {
  const tasks = [
    // ...
  ]
  let currentTaskIndex = 0
 
  while (currentTaskIndex < tasks.length) {
    const currentTask = tasks[currentTaskIndex]
 
    // process task
    // ...
 
    currentTaskIndex++
  }
}

Examples

Incorrect Examples

for (let i = 0; i !== arr.length; i++) {
  // this is bad because it should use `i < arr.length` to be safer
}

for (let i = arr.length; i !== -1; i--) {
  // this is bad because it should use `i >= 0` or `i > -1` to be safer
}

while (i !== tasks.length) {
  // this is bad because it should use `i < tasks.length` to be safer
  ++i
}

const length = arr.length
let i = 0
while (i !== length) {
  // this is bad because the while loop should use `i < length` to be safer
  ++i
}

Correct Examples

for (let i = 0; i < arr.length; i++) {}

for (let i = arr.length; i >= 0; i--) {}

while (i < tasks.length) {
  ++i
}

Metadata

Key	Value
name	`prefer-loose-array-bounds-checks-in-loops`
level	`error`
scope	`file`
fixable	false
cacheable	true
tags	[ `best practices` ]
exclude	[ `*/.test\.{js,ts,jsx,tsx,cjs,mjs}` ]
gritql	true

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